Stainless Steel, Precipitation Hardening
630 Stainless Steel (S17400) Bar
Martensitic precipitation hardening stainless steel with 17% chromium and 4% nickel.
The most well known precipitation hardening steel is 17-4. The name comes from the additions 17% chromium and 4% nickel. It also contains 4% copper and 0.3% niobium. 17-4 is also known as stainless steel grade 630.
Precipitation hardening stainless steels are chromium and nickel containing steels that provide an optimum combination of the properties of martensitic and austenitic grades. Like martensitic grades, they are known for their ability to gain high strength through heat treatment and they also have the corrosion resistance of austenitic stainless steel.
The high tensile strengths of precipitation hardening stainless steels come after a heat treatment process that leads to precipitation hardening of a martensitic or austenitic matrix. Hardening is achieved through the addition of one or more of the elements copper, aluminium, titanium, niobium, and molybdenum.
The advantage of precipitation hardening steels is that they can be supplied in a “solution treated” condition, which is readily machinable. After machining or another fabrication method, a single, low temperature heat treatment can be applied to increase the strength of the steel. This is known as ageing or age-hardening. As it is carried out at low temperature, the component undergoes no distortion.
DOWNLOAD PDF
Range
| Product Form | Condition | Imperial Sizes | Metric Sizes |
| Round Bar | Condition A | 3⁄8" - 12" | 45mm |
| Round Bar | Condition H1150D | 3⁄8" - 12" | 45mm |
PLEASE NOTE
If you do not see what you are looking for, please contact your local service centre with your specific requirements.
630 Stainless Steel Related Specifications
| System / Standard | Country / Region | Grade / Designation |
| AISI | USA | 630 (17-4PH) |
| UNS | International | S17400 |
| EN / DIN | Europe | 1.4542 |
| EN Name | Europe | X5CrNiCuNb16-4 |
| ASTM A564 | USA | Grade 630 (bars, forgings) |
| ASTM A693 | USA | Grade 630 (plate, sheet) |
| ASTM A705 | USA | Grade 630 (forgings) |
| AMS | USA / Aerospace | AMS 5643 / 5604 (17-4PH) |
| GB | China | 0Cr17Ni4Cu4Nb |
| JIS | Japan | SUS630 |
| BS | UK | 17/4PH |
| AFNOR | France | Z6CNU17-04 |
Properties
Chemical Composition
| Chemical Element | % Present |
| Carbon (C) | 0.00 - 0.07 |
| Chromium (Cr) | 15.00 - 17.50 |
| Manganese (Mn) | 0.00 - 1.00 |
| Silicon (Si) | 0.00 - 1.00 |
| Phosphorous (P) | 0.00 - 0.04 |
| Sulphur (S) | 0.00 - 0.03 |
| Nickel (Ni) | 3.00 - 5.00 |
| Copper (Cu) | 3.00 - 5.00 |
| Molybdenum (Mo) | 0.00 - 0.50 |
| Niobium (Columbium) (Nb) | 0.00 - 0.45 |
| Columbium (Cb) | 0.00 - 0.45 |
| Iron (Fe) | Balance |
Mechanical Properties
| Condition | Tensile Strength (MPa) | 0.2% Yield Strength (MPa) | Elongation (%) | Hardness (HRC) |
| Annealed (Solution Treated) | ~860–930 (typical, not minimum design value) | ~520–620 (typical) | ~18–22 | ~25–30 |
| H900 | ≥ 1,310 | ≥ 1,175 | ≥ 10 | ~ 40–44 |
| H1025 | ≥ 1,100 | ≥ 1,000 | ≥ 12 | ~ 35–38 |
| H1075 | ≥ 1,035 | ≥ 965 | ≥ 13 | ~ 32–36 |
| H1150 | ≥ 930 | ≥ 725 | ≥ 16 | ~ 28–32 |
| H1150M / Double H1150 | ~ 930 (typical) | ~ 690–725 (typical) | ~ 16–18 | ~ 26–30 |
General Physical Properties
| Physical Property | Value |
| Density | 7.75 g/cm³ |
| Thermal Expansion | 10.8 x 10-6/K |
| Modulus of Elasticity | 196 GPa |
| Thermal Conductivity | 18.4 W/m.K |
| Electrical Resistivity | 0.8 x 10-6 Ω .m |
Applications of 630 (17-4PH) Stainless Steel
630 stainless steel (also known as 17-4PH, SUS630, 0Cr17Ni4Cu4Nb) is a martensitic precipitation-hardening stainless steel that combines high strength, good corrosion resistance and excellent toughness. It is widely used where high mechanical performance and stainless capability are required in one material.
1. High-Strength Shafts and Rotating Components
Pump and compressor shafts in water, steam and mildly corrosive process media
Drive shafts, rotor shafts and spindle components in industrial machinery
Agitator, mixer and stirrer shafts in chemical, paper and food-processing equipment
Power-transmission parts that must carry high torque with good fatigue strength
2. Aerospace and Defense Components
Actuator parts, landing gear pins, bushes and fittings in aircraft systems
Structural brackets, hardware and linkages requiring high strength-to-weight ratio
Missile, weapon and defence-system parts where reliability and corrosion resistance are critical
Components operating in humid or mildly corrosive environments with high cyclic loads
3. High-Strength Fasteners and Mechanical Fixings
High-strength bolts, screws and studs for aerospace, energy and marine service
Pins, dowels, clevis pins and axles used in outdoor or corrosive conditions
Fasteners that need higher strength than 304/316 while still resisting rust and staining
Mechanical joints where preload retention and resistance to relaxation are important
4. Valves, Pumps and Fluid-Control Hardware
Valve bodies, stems, seats and internal trim for water, oil, gas and process fluids
Pump impellers, casings and wear rings exposed to both pressure and corrosion
Fittings, couplings and manifolds in chemical, petrochemical and power plants
Components requiring a combination of sealing integrity, wear resistance and stainless performance
5. Moulds, Tooling and Precision Industrial Parts
Plastic mould tools and inserts needing corrosion resistance to cooling water and moulding gases
Tooling and fixtures used in corrosive shop environments where carbon steel would rust
Precision mechanical parts that require high strength, dimensional stability and good surface finish
Gears, rings and coupling elements where wear and fatigue resistance are important
6. Marine and Offshore Equipment
Propulsion and steering components, such as propeller hubs and shaft sleeves
Deck hardware, winch parts and lifting components exposed to seawater spray and marine atmospheres
Subassemblies that require better corrosion resistance than carbon steel and higher strength than standard austenitic stainless steels
Hardware where weight saving and compact design are beneficial in marine systems
Summary
630 (17-4PH) stainless steel is mainly used for high-strength shafts and rotating parts, aerospace and defence components, high-performance fasteners, valves and pump parts, moulds and tooling, and marine or industrial hardware that must combine high strength, good toughness and reliable corrosion resistance in one versatile stainless steel grade.
Characteristics of 630 (17-4PH) Stainless Steel
630 stainless steel (also known as 17-4PH, SUS630, 0Cr17Ni4Cu4Nb) is a martensitic precipitation-hardening stainless steel designed to provide a combination of high strength, good toughness and useful corrosion resistance in a single, versatile alloy.
1. Precipitation-Hardening Martensitic Stainless Steel
17-4PH is a Cr–Ni–Cu–Nb precipitation-hardening martensitic stainless steel.
Strength is developed by solution treatment followed by aging (“H” conditions such as H900, H1025, H1150).
The microstructure after aging is a tempered martensite matrix strengthened by finely dispersed precipitates.
2. High Strength and Hardness After Aging
630 can achieve very high strength levels compared with standard stainless steels.
In high-strength conditions (e.g. H900), it offers yield strengths well above typical structural steels.
Hardness and tensile strength can be adjusted via the aging temperature, providing a wide property range from very high strength to more ductile, tougher conditions.
3. Good Corrosion Resistance
Corrosion resistance is significantly better than that of conventional martensitic grades such as 410 / 420.
In many atmospheric, fresh-water and mildly corrosive industrial environments, 17-4PH performs reliably with low risk of rusting or pitting.
Its corrosion resistance is generally below that of 316 in very aggressive chloride or chemical environments, but adequate for many mechanical, marine and process applications.
4. Toughness and Fatigue Performance
When aged at appropriate temperatures (e.g. H1025, H1075, H1150), 630 offers a good balance of strength and toughness.
It exhibits reliable fatigue performance for shafts, fasteners and rotating components under cyclic loading.
Higher aging temperatures reduce strength but improve toughness and damage tolerance, which is important for safety-critical components.
5. Heat Treatment Flexibility and Dimensional Stability
17-4PH is supplied solution treated and is then aged to the required condition.
Aging is done at relatively low temperatures, giving good dimensional stability and allowing close tolerances after final heat treatment.
Different aging conditions (H900, H1025, H1075, H1150, H1150M) allow designers to optimise strength, toughness and stress-corrosion performance for specific applications.
6. Machinability and Surface Finish
Machinability is generally better than many high-alloy tool steels, especially in the solution-treated or softer aged conditions.
Using appropriate tooling and cutting parameters, 630 can be turned, milled and drilled to produce accurate, high-quality surfaces.
After aging, it can be ground and polished to excellent surface finishes for shafts, seals, valve components and precision mechanical parts.
7. Weldability and Fabrication
17-4PH can be welded with suitable procedures and filler metals, usually in the solution-treated condition.
Post-weld aging restores high strength and helps equalise properties across weld metal, heat-affected zone and base material.
With correct control of heat input, preheat/interpass temperatures and post-weld treatment, sound welds with good mechanical and corrosion properties can be achieved.
8. Magnetic Properties
Because it is a martensitic precipitation-hardening stainless steel, 630 is strongly magnetic in all aged conditions.
This is important for applications involving magnetic clamping, sensing or assembly in contrast to non-magnetic austenitic grades such as 304 / 316.
Summary
630 (17-4PH) stainless steel is a martensitic precipitation-hardening stainless alloy that combines high, adjustable strength and hardness, good corrosion resistance, useful toughness and fatigue performance, dimensional stability during aging, reasonable machinability and strong magnetism, making it a widely used choice for high-strength shafts, fasteners, valves, pump components, aerospace and marine hardware and many other demanding mechanical applications.
Additional Information
Weldability
Weldability of 630 (17-4PH) Stainless Steel
630 stainless steel (17-4PH, SUS630, 0Cr17Ni4Cu4Nb) is weldable, but as a high-strength precipitation-hardening martensitic stainless steel it requires more control than 304/316. With the correct procedures and post-weld heat treatment, sound welds with high strength and good corrosion resistance can be obtained.
1. General Weldability Characteristics
630 can be welded by most common fusion welding processes.
It should always be treated as a high-strength, crack-sensitive alloy, not as an austenitic stainless steel.
Improper welding (no PWHT, excessive heat, poor technique) can reduce toughness, promote cracking and lower corrosion resistance in the weld and heat-affected zone.
2. Recommended Welding Condition of the Base Metal
Welding is usually performed in the solution-treated (annealed) condition.
After welding, the whole part or assembly is aged to the required condition (e.g. H900, H1025, H1150).
Welding in a fully aged, high-strength condition greatly increases cracking risk and is generally avoided except for minor, non-critical work.
3. Suitable Welding Processes
Typical processes for 17-4PH include:
GTAW (TIG) – preferred for high-quality, low-heat-input welds and thin sections.
GMAW (MIG) – suitable for production welding of thicker sections with proper shielding and parameter control.
SMAW (MMA) – possible using low-hydrogen electrodes for repair and site work.
Laser or electron-beam welding – used for precision, low-distortion joints in critical components.
Process choice depends on thickness, joint design, accessibility and quality requirements.
4. Filler Metal Selection
Filler selection depends on whether matching strength or maximum toughness is required:
Matching / near-matching filler (17-4PH-type wire or electrodes) is used when weld strength must closely match the base metal and the joint will be aged.
In some cases, austenitic stainless fillers (e.g. 309L/308L) may be used to improve weld toughness and reduce cracking tendency, especially for dissimilar joints or highly restrained welds, but weld strength will be lower than the aged base metal.
For critical applications, filler wires and procedures should conform to relevant codes and be supported by procedure qualification records (PQR).
5. Preheat and Interpass Temperature Control
Preheat requirements for 17-4PH are moderate but important:
A modest preheat is often recommended for thicker sections to reduce thermal gradients and risk of hydrogen cracking.
Maintain a reasonable interpass temperature, avoiding both very cold starts and excessive heat build-up.
Excessive heat input or high interpass temperatures can coarsen the microstructure and distort the subsequent aging response, reducing toughness.
Good temperature control helps achieve uniform properties and minimise distortion.
6. Post-Weld Heat Treatment and Aging
Post-weld heat treatment is critical to restoring strength and toughness:
Welds are typically made in the solution-treated condition, then the entire component is aged to the specified condition (H900, H1025, H1075, H1150, etc.).
Aging brings the weld metal, heat-affected zone and base metal to a consistent precipitation-hardened state, giving near-uniform strength across the joint.
For some less critical welds, a higher-temperature aging condition (e.g. H1150) may be chosen to improve toughness and stress-corrosion performance at the expense of some strength.
Where design allows, full solution treatment plus aging after welding provides the most consistent properties.
7. Crack Control, Distortion and Joint Design
To minimise cracking and distortion:
Use low-hydrogen procedures: dry consumables, clean joint preparation, correct shielding gas and adequate gas coverage.
Avoid sharp corners, abrupt section changes and highly restrained joints that concentrate stresses at the weld.
Use appropriate joint fit-up and tack welding to control distortion, especially on long shafts or thin-wall sections.
For critical welds, apply non-destructive examination (PT, MT, UT or RT as appropriate) to verify weld soundness.
Careful joint design and weld sequencing help maintain alignment and dimensional accuracy after aging.
8. Effect of Welding on Mechanical and Corrosion Properties
Welding can affect both strength and corrosion behaviour if not controlled:
Without proper PWHT, welds may have heterogeneous microstructures, leading to variations in hardness, reduced impact toughness and lower fatigue performance.
Incorrect fillers or procedures can reduce corrosion resistance and increase susceptibility to stress-corrosion cracking, especially in chloride environments.
Correctly welded and aged 17-4PH joints can achieve properties close to those of the parent metal, with good strength, toughness and corrosion resistance suitable for demanding mechanical and structural applications.
Summary
630 (17-4PH) stainless steel is weldable but must be treated as a high-strength precipitation-hardening martensitic alloy: reliable welds require welding in the solution-treated condition, appropriate filler selection, controlled heat input and interpass temperature, followed by aging (and, where required, solution treatment) so that the weld metal, heat-affected zone and base metal all achieve the specified strength, toughness and corrosion resistance for critical service.
Fabrication
Fabrication of 630 (17-4PH) Stainless Steel
630 stainless steel (17-4PH, SUS630, 0Cr17Ni4Cu4Nb) is a precipitation-hardening martensitic stainless steel. It can be forged, machined and welded successfully, but fabrication must always be planned together with the heat treatment and aging schedule to control strength, toughness and dimensional stability.
1. General Fabrication Approach
630 is usually supplied in the solution-treated condition and then aged to the required “H” condition.
Major forming, forging and rough machining are best done before final aging, with only light finishing operations afterwards.
Because the alloy can reach very high strength, fabrication routes must consider distortion control, residual stresses and final tolerances from the start.
2. Forming and Cold Working
Cold formability is limited compared with austenitic stainless steels.
Light operations such as straightening, gentle bending with large radii and sizing can be carried out in the solution-treated condition.
Severe cold forming, tight-radius bending or deep drawing are generally not recommended, especially in hardened conditions such as H900.
If significant cold work is unavoidable, a subsequent solution treatment and re-aging (or at least stress relief) is recommended to restore toughness and dimensional stability.
3. Hot Working and Forging
630 can be hot worked and forged using standard stainless and high-alloy practices.
Forging is performed in an appropriate high-temperature range, followed by air cooling and then solution treatment to set a uniform martensitic structure.
Adequate reductions and proper temperature control help produce a fine, even grain size, which improves toughness and fatigue performance.
After hot working and solution treatment, parts are ready for rough machining and later aging to the specified condition (H900, H1025, H1150, etc.).
4. Machining
Machinability of 17-4PH is moderate for a high-strength stainless alloy.
Rough machining is best performed in the solution-treated or softer aged condition, where cutting forces and tool wear are manageable.
After aging to final hardness, only light finishing cuts or grinding should be used to achieve final dimensions and surface finish.
Rigid fixturing, sharp carbide tools, conservative cutting speeds, adequate feed and generous coolant are important for good tool life and accurate, clean surfaces.
5. Heat Treatment Within the Fabrication Route
Heat treatment is central to 630 fabrication:
A typical sequence is: hot working (if any) → solution treatment → rough machining → aging to required condition → finish machining and grinding.
Aging at relatively low temperatures provides dimensional stability, which is useful for tight-tolerance components.
Different aging conditions (H900, H1025, H1075, H1150, H1150M) are chosen to balance strength, toughness and stress-corrosion performance for the final application.
6. Welding as Part of Fabrication
Welding is normally done in the solution-treated condition, followed by aging for the entire assembly.
This approach helps ensure that weld metal, heat-affected zone and base material all develop consistent strength and corrosion resistance.
Low-hydrogen procedures, controlled heat input and suitable fillers (matching or austenitic, depending on strength and toughness requirements) are essential to minimise cracking and preserve properties.
7. Dimensional Stability, Grinding and Surface Finishing
Because 630 gains high strength by martensitic transformation and aging, distortion control is important.
Good practice includes: rough machining before final aging, allowing for small movements during heat treatment, and then finish machining or grinding after aging.
The alloy can be ground and polished to very high-quality surfaces, which is important for shafts, bearing seats, sealing faces, valve components and precision mechanical parts.
Proper removal of scale, oxides and machining marks also helps maximise fatigue and corrosion performance.
Summary
630 (17-4PH) stainless steel can be fabricated into high-strength, high-reliability components when forming, forging, machining, welding and heat treatment are coordinated as one route—doing major work in the solution-treated condition, then aging and finishing to achieve the required strength, toughness, dimensional accuracy and surface quality for demanding mechanical and structural applications.
Hot Working
Hot Working of 630 (17-4PH) Stainless Steel
630 stainless steel (17-4PH, SUS630, 0Cr17Ni4Cu4Nb) is a precipitation-hardening martensitic stainless steel that can be forged and hot worked successfully when temperature, reduction and cooling are properly controlled. Correct hot-working practice is important to obtain a fine, uniform structure before solution treatment and aging.
1. Recommended Hot Working Temperature Range
Hot working and forging are normally carried out in a moderate high-temperature range to balance ductility and grain control.
Typical heating / forging range is about 950–1,050°C (1,740–1,920°F).
Deformation should begin towards the upper end of this range for best plasticity.
Working should stop above roughly 870–900°C (1,600–1,650°F) to avoid cracking as ductility falls at lower temperatures.
Exact temperatures should follow the relevant mill or material specification for the product form.
2. Forging and Reheating Practice
Before deformation, the section should be heated uniformly through to the target temperature.
Use firm, substantial reductions per pass rather than light tapping to promote good grain refinement.
For large forgings or complex shapes, return the part to the furnace as soon as its temperature approaches the lower limit of the working range.
Avoid prolonged soaking at the highest temperatures, which can cause grain growth and excessive scale formation.
Well-controlled forging sets up the microstructure for later solution treatment and aging.
3. Cooling After Hot Working
After forging or hot forming, parts are normally cooled in still air or controlled conditions.
Hot working is usually followed by a full solution treatment (annealing) to develop a uniform martensitic structure.
After solution treatment, the material can then be aged to the required condition (H900, H1025, H1075, H1150, etc.) to achieve the specified strength and toughness.
Very slow furnace cooling through the transformation range should be avoided when high, uniform mechanical properties are required.
4. Surface Protection and Scale Removal
At forging temperatures, 17-4PH will form oxide scale and may experience some surface decarburisation.
Allow sufficient machining or grinding allowance to remove scale and any affected surface layer after hot working.
Where practical, use controlled atmospheres or protective coatings in furnaces to reduce scale formation on critical surfaces.
After forging and before final machining, apply shot blasting, grinding or rough machining to restore a clean, sound surface.
5. Influence on Microstructure and Mechanical Properties
Adequate hot reduction in the correct temperature range promotes a fine, uniform grain size.
A refined grain structure improves toughness, fatigue performance and consistency of properties through the section.
Insufficient deformation, over-heating or working over too wide a temperature range can leave coarse or non-uniform grains, which may reduce toughness, especially in transverse directions.
A subsequent solution treatment plus aging is essential to reset the microstructure and develop the precipitation-hardening response.
6. Distortion, Cracking Control and Design Considerations
Preforms and forgings should be designed with smooth transitions and reasonably uniform section thickness to reduce internal stresses.
Avoid sharp corners, sudden changes in cross-section and heavy localised reductions, which can promote cracking during forging or cooling.
For long shafts or complex shapes, consider intermediate stress relief or normalising steps if very heavy reductions are applied.
Inspect forgings for laps, folds and surface or internal cracks before committing to final heat treatment and machining to minimise scrap risk and rework.
Summary
Hot working of 630 (17-4PH) stainless steel is best carried out in a controlled range around 950–1,050°C with uniform heating, substantial reductions and air cooling, followed by solution treatment and aging; careful control of temperature, deformation and post-forging cleanup is essential to obtain a fine, uniform microstructure, minimise defects and deliver consistent high-strength properties in the finished components.
Heat Resistance
Heat Resistance of 630 (17-4PH) Stainless Steel
630 stainless steel (17-4PH, SUS630, 0Cr17Ni4Cu4Nb) offers good heat resistance for a high-strength precipitation-hardening martensitic stainless steel. It retains useful mechanical properties at moderately elevated temperatures, but it is not a dedicated high-temperature or creep-resistant alloy.
1. Service Temperature Range
630 is generally used at room temperature up to about 300–315°C (≈570–600°F) for continuous service.
Within this range, it maintains a favourable combination of high strength, hardness and toughness.
Long-term service significantly above this band is not recommended, as overaging and loss of strength become more pronounced.
2. Influence of Aging Condition on Heat Resistance
The chosen aging condition (H900, H1025, H1075, H1150, H1150M) strongly affects elevated-temperature behaviour:
H900 – maximum strength at room temperature, more sensitive to overaging and toughness loss at elevated temperature.
H1025 / H1075 – slightly lower strength but better toughness and more stable behaviour with moderate heat.
H1150 / H1150M – lowest strength but best toughness and stress-corrosion performance; often more tolerant of moderate temperature exposure.
In design, the continuous service temperature should be kept comfortably below the aging temperature used for the material.
3. Strength and Toughness at Elevated Temperature
As temperature rises, 630 behaves like other steels:
Tensile and yield strength decrease with increasing temperature.
Fatigue strength under cyclic loading is reduced.
Impact toughness may fall, particularly in the highest-strength (H900) condition.
Within its recommended temperature range, however, 17-4PH still provides higher strength than standard austenitic grades and many conventional martensitic steels.
4. Oxidation and Surface Behaviour
With about 15–17% Cr plus Ni and Cu, 630 has better oxidation resistance than carbon or low-alloy steels at moderate temperatures:
Forms a protective oxide film in air under typical service conditions.
For prolonged high-temperature exposure, its oxidation resistance is lower than that of dedicated heat-resistant austenitic grades, especially at temperatures well above 600°F.
Maintaining smooth, clean surfaces and avoiding heavy scale formation helps preserve fatigue performance and corrosion resistance.
5. Overaging and Property Degradation
Exposure near or above the aging temperature for extended periods can:
Over-age the precipitates, reducing strength and hardness.
Alter the microstructure, affecting fatigue performance and stress-corrosion behaviour.
Gradually shift the strength–toughness balance away from the originally specified condition.
For critical components, design allowable stresses should consider potential overaging if service temperatures approach the material’s aging temperature over long periods.
6. Comparison with Other Stainless Steels and High-Temperature Alloys
Compared with other stainless steels:
Versus 410 / 420: 630 offers much higher strength and generally better heat and corrosion resistance in moderate-temperature service.
Versus 304 / 316: 630 has far higher room-temperature strength, but inferior long-term high-temperature strength, creep resistance and oxidation resistance for continuous service at higher temperatures.
Versus special heat-resistant austenitic or nickel alloys: 630 is not a replacement where creep, scale resistance and very high service temperatures are the main design drivers.
It is best viewed as a high-strength structural stainless with good moderate-temperature capability, not as a primary high-temperature alloy.
Summary
630 (17-4PH) stainless steel provides reliable heat resistance for structural and mechanical components operating at moderate temperatures (typically up to about 300–315°C / 570–600°F), retaining high strength and useful toughness with good oxidation behaviour; however, prolonged exposure above this range leads to overaging and loss of properties, so it should be treated as a high-strength stainless steel with limited high-temperature capability rather than a dedicated heat-resistant or creep-resistant alloy.
Machinability
Machinability of 630 (17-4PH) Stainless Steel
630 stainless steel (17-4PH, SUS630, 0Cr17Ni4Cu4Nb) is a high-strength precipitation-hardening martensitic stainless steel with moderate machinability. It is generally more difficult to machine than 304/316, but easier than many tool steels when machined in the correct condition with suitable tooling and parameters.
1. General Machining Behaviour
In the solution-treated or softer aged conditions, 17-4PH machines reasonably well for a high-strength alloy.
It does not work harden as severely as austenitic grades, but its higher base strength means greater cutting forces and tool wear.
Best practice is to treat 630 as a high-strength alloy steel, not as a free-machining stainless, and to plan machining routes accordingly.
2. Preferred Conditions for Machining
Rough machining is best carried out in the solution-treated (annealed) condition or in a higher-temperature aged, lower-strength condition (e.g. H1075/H1150).
After roughing, parts are aged to the required condition (H900, H1025, H1150, etc.), followed by light finishing cuts or grinding.
Machining directly in the highest-strength condition (H900) is possible but leads to shorter tool life, higher heat and lower productivity, so it is normally limited to finishing only.
3. Tooling and Cutting Parameters
Carbide tooling is recommended for most turning, milling and drilling operations.
Good practice includes:
Using tool grades designed for stainless or hardened steels
Running moderate cutting speeds with sufficient feed to avoid rubbing and surface glazing
Employing positive rake, rigid toolholders and stiff setups to minimise chatter and edge chipping
Avoiding very light, rubbing cuts that generate heat without effectively removing material
Correct choice of insert geometry and grade significantly improves tool life and surface finish.
4. Coolant and Chip Control
Effective coolant use is important to manage heat and extend tool life:
Apply plenty of cutting fluid or coolant at the cutting zone during turning, milling and drilling
Use coolant both for temperature control and for chip evacuation in deep holes or enclosed cuts
630 can produce tough, continuous chips, especially in softer conditions, so:
Use chip-breaker geometries on inserts
Optimise feed and depth of cut to promote chip breaking
Ensure safe chip control in automatic or high-speed operations to avoid surface damage and machine stoppages
5. Drilling, Tapping and Threading
For drilling:
Use high-quality cobalt HSS or carbide drills
Employ moderate speeds with steady feed and peck cycles on deeper holes to clear chips
For tapping and threading:
Use strong, premium taps with plenty of lubrication, especially in harder conditions
Where possible, prefer thread milling for critical or high-strength parts to reduce the risk of tap breakage
Allow for some spring-back in high-strength material when selecting thread tolerances and final dimensions
Good hole preparation (correct size, chamfering and alignment) helps reduce tool stress and improve thread quality.
6. Surface Finish and Dimensional Control
630 can be finished to very high surface quality by turning, grinding and polishing, which is essential for:
Shafts and bearing seats
Valve stems and sealing surfaces
Precision mechanical components and mating fits
To maintain dimensional accuracy and surface integrity:
Plan a route of rough machining → heat treatment (aging) → finish machining / grinding
Use light finishing passes with sharp tools after aging, particularly on tight-tolerance diameters
Avoid overheating the surface during machining or grinding to prevent local tempering, micro-cracking or residual tensile stresses
Careful fixturing and balanced machining operations help minimise distortion, especially on long or slender parts.
Summary
The machinability of 630 (17-4PH) stainless steel is moderate: it machines best in the solution-treated or softer aged conditions using rigid setups, carbide tooling, conservative cutting speeds, effective coolant and good chip control, followed by light finishing or grinding after aging to achieve accurate dimensions and high-quality surfaces on high-strength shafts, fasteners, valve components and other precision parts.
Corrosion Resistance
Corrosion Resistance of 630 (17-4PH) Stainless Steel
630 stainless steel (17-4PH, SUS630, 0Cr17Ni4Cu4Nb) offers good corrosion resistance for a high-strength martensitic precipitation-hardening stainless steel. In many environments it performs clearly better than standard martensitic grades such as 410/420, but generally below 316 in very aggressive chloride or chemical conditions.
1. General Corrosion Behaviour
630 provides a balanced combination of high strength and stainless corrosion resistance.
It resists uniform corrosion in many atmospheric, fresh-water and mildly industrial environments.
The Cr–Ni–Cu–Nb composition gives better corrosion performance than plain martensitic steels, while still allowing high strength through aging.
2. Atmospheric and Fresh-Water Environments
In typical outdoor and indoor atmospheres, 17-4PH has good resistance to rusting and staining.
It performs well in fresh water, cooling water and many process waters where chloride levels are low to moderate.
This makes it suitable for shafts, fasteners, valve and pump components exposed to weather, humidity, splash and wash-down conditions.
3. Marine and Chloride-Containing Environments
In marine and chloride-bearing environments, 630 behaves as a “good but not extreme” stainless grade.
It offers better corrosion performance than carbon steel and conventional martensitic stainless steels in marine atmospheres and splash zones.
However, its pitting and crevice corrosion resistance is lower than that of molybdenum-rich austenitic and duplex grades (such as 316 or duplex 2205), especially in hot, concentrated or stagnant chloride solutions.
Continuous immersion in seawater, particularly at elevated temperatures, is not ideal for the highest reliability applications.
4. Behaviour in Chemical Process Media
630 is suitable for many mildly to moderately corrosive chemical environments.
It generally performs well in light acids, alkaline solutions and many organic fluids at controlled temperatures and concentrations.
It is widely used in process plant components where both high strength and stainless behaviour are required.
It is not recommended for strong mineral acids, strong reducing acids or hot, concentrated chlorides, where more highly alloyed stainless steels or nickel alloys are preferable.
5. Stress-Corrosion Cracking and Hydrogen Effects
As a high-strength stainless steel, 17-4PH is more sensitive to stress-corrosion cracking (SCC) than lower-strength austenitic grades.
Risk is greatest in chloride-containing environments under sustained tensile stress, especially at elevated temperature.
Hydrogen-charging conditions (acid pickling, electroplating, excessive cathodic protection) can also promote hydrogen embrittlement or delayed cracking if not properly controlled.
Good practice includes minimising residual stresses, avoiding unnecessary high-strength conditions in severe environments, and carefully managing any processes that introduce hydrogen.
6. Influence of Heat Treatment, Surface Finish and Design
Corrosion resistance of 630 is strongly influenced by heat treatment, surface condition and component design.
Correct solution treatment and aging give a refined microstructure and consistent corrosion behaviour.
Smooth, polished surfaces resist pitting and crevice attack better than rough or damaged surfaces.
Proper cleaning to remove scale, weld spatter, embedded iron and machining contamination is essential for optimum performance.
Good design avoids tight crevices, stagnant pockets and water traps, and ensures welds are well contoured, finished and cleaned.
Summary
630 (17-4PH) stainless steel offers good overall corrosion resistance in atmospheric, fresh-water, mildly marine and many industrial environments, clearly superior to standard martensitic grades and adequate for a wide range of high-strength mechanical applications, but it does not match the chloride or chemical resistance of highly alloyed austenitic and duplex stainless steels in the most aggressive service conditions.
Heat Treatment
Heat Treatment of 630 (17-4PH) Stainless Steel
630 stainless steel (17-4PH, SUS630, 0Cr17Ni4Cu4Nb) is a precipitation-hardening martensitic stainless steel. Its final mechanical properties are controlled by solution treatment followed by aging (“H” conditions such as H900, H1025, H1150), so heat treatment design is a key part of component engineering and fabrication.
1. Objectives of Heat Treatment
Develop a martensitic matrix suitable for precipitation hardening
Achieve high tensile and yield strength with controlled hardness
Adjust toughness, fatigue performance and stress-corrosion resistance by choosing appropriate aging conditions
Minimise residual stress and distortion from forging, machining and welding
2. Solution Treatment (Annealing / Austenitizing)
630 is first solution treated to dissolve alloying elements and homogenise the structure:
Heated to the specified austenitizing temperature (typically in the 1,000–1,050°C range, depending on spec)
Held long enough to ensure uniform temperature and composition through section thickness
Rapidly cooled (usually air or oil, depending on section size and requirement) to form a predominantly martensitic structure
In this condition the material has moderate strength, reasonable machinability and is ready for aging or further fabrication
3. Aging / Precipitation Hardening Conditions
After solution treatment, 17-4PH is aged to develop its final properties. Common conditions include:
H900 – lower aging temperature, maximum strength and hardness, reduced toughness
H1025 / H1075 – slightly lower strength than H900 but improved toughness and fatigue performance
H1150 / H1150M – lowest strength, highest toughness and best stress-corrosion cracking resistance
Aging is done by reheating to the chosen temperature, holding for a specified time and cooling in air, allowing fine precipitates to form and strengthen the martensitic matrix.
4. Effect of Aging Temperature on Properties
Aging temperature controls the balance of strength and toughness:
Lower aging temperatures (H900) → very high yield and tensile strength, high hardness, lower impact toughness
Intermediate aging (H1025, H1075) → reduced hardness and strength, but better toughness and fatigue resistance
Higher aging temperatures (H1150, H1150M) → moderate strength, maximum toughness and improved resistance to stress-corrosion cracking
Designers select the condition according to whether peak strength or damage tolerance/environmental resistance is the primary requirement.
5. Stress Relief and Post-Weld Heat Treatment
Stress-relief and post-weld treatments are often integrated into the heat-treatment plan:
Heavily machined or straightened parts may receive a sub-critical stress-relief before final aging to reduce distortion risk.
Welded assemblies are usually welded in the solution-treated condition and then aged as a whole so that weld metal, heat-affected zone and base metal reach a consistent strength level.
For the most demanding applications, some routes use solution treatment + aging after welding to fully reset the microstructure and properties.
6. Typical Production Heat-Treatment Sequences
For many 17-4PH components, practical routes include:
Forged parts: forge → air cool → solution treat → rough machine → age (H900/H1025/H1150) → finish machine / grind
Welded fabrications: solution-treated material → weld with approved procedure → age to required condition → final machining / finishing
Precision components: solution treat → rough machine → age → light finish machining / grinding to final size and surface finish
These sequences help balance machinability, distortion control and final property requirements.
7. Precautions During Heat Treatment
Avoid overheating during solution treatment to prevent grain coarsening and toughness loss
Ensure accurate furnace temperature control and adequate soak time for thicker sections
Protect surfaces against scaling and decarburisation, or allow machining allowance to remove affected layers
Do not exceed specification limits on repeated solution and aging cycles to avoid property degradation
Maintain proper support and fixturing during heating and cooling to minimise distortion, especially for long, slender parts
Summary
Heat treatment of 630 (17-4PH) stainless steel is based on solution treatment to form a martensitic structure followed by aging at controlled temperatures (H900, H1025, H1075, H1150, etc.) to tune strength, hardness, toughness and stress-corrosion performance; by integrating stress relief and post-weld aging into the manufacturing route, engineers can produce high-strength, dimensionally stable components with properties matched to demanding mechanical and environmental service.
Cold Working
Cold Working of 630 (17-4PH) Stainless Steel
630 stainless steel (17-4PH, SUS630, 0Cr17Ni4Cu4Nb) is a high-strength precipitation-hardening martensitic stainless steel with limited cold workability compared with austenitic grades such as 304/316. Cold working is possible, but it should generally be kept to light or moderate deformation and coordinated with the heat treatment schedule.
1. General Cold Workability
630 has lower ductility than austenitic stainless steels, especially in high-strength aged conditions such as H900.
It can accept modest cold deformation for straightening, sizing and small geometry adjustments.
Heavy cold forming, tight-radius bending or deep drawing are not recommended, particularly on thick sections or fully hardened material.
2. Preferred Condition for Cold Working
Cold working should be carried out mainly in the solution-treated (annealed) condition or in a softer, higher-temperature aged condition (e.g. H1075 or H1150).
In these conditions, ductility is higher and the risk of cracking or excessive work hardening is reduced.
In high-strength conditions (such as H900), cold work should be limited to very small adjustments only, such as slight straightening or minor bending.
3. Typical Cold Working Operations
Practical cold working operations for 630 include:
Straightening bars, shafts and pins after heat treatment or machining.
Gentle bending with generous bend radii in the solution-treated condition.
Light sizing, swaging or reduction of diameter where total strain is kept modest.
Operations generally unsuitable for 17-4PH (except possibly on thin sections) include:
Severe cold heading with large upset ratios.
Tight-radius bending of thick flats, bars or plates.
Complex deep drawing and heavy press-forming.
4. Effects on Properties and Residual Stresses
Cold work increases local strength and hardness, but reduces toughness and ductility in highly strained zones.
Residual stresses introduced by bending, straightening or swaging can influence fatigue performance, dimensional stability and stress-corrosion behaviour.
Because 630 already relies on a controlled precipitation-hardened martensitic microstructure, uncontrolled cold deformation can make properties less uniform through the cross-section.
5. Stress Relief and Heat Treatment After Cold Work
After significant cold work, some form of heat treatment is recommended:
For major deformation, a full solution treatment followed by re-aging is often the best way to restore a uniform microstructure and consistent properties.
For moderate adjustments in an already aged condition, a sub-critical stress-relief treatment can reduce residual stresses without drastically changing strength.
Critical components (e.g. high-load shafts, fasteners, aerospace fittings) should not rely on heavily cold-worked, unrelieved material in service.
6. Design and Process Recommendations
Plan forming operations so that most shaping is done before final aging.
Use larger bend radii and gradual transitions to reduce local strain and avoid cracking.
Avoid sharp corners, notches and abrupt cross-section changes in areas that will be cold worked.
For tight tolerances, a typical route is: rough shaping and light cold work → solution treatment and aging → finish machining and grinding to final size.
Summary
Cold working of 630 (17-4PH) stainless steel should be limited to light to moderate operations such as straightening, sizing and gentle bending carried out mainly in the solution-treated or softer aged conditions; heavier deformation can reduce toughness and introduce harmful residual stresses, so significant cold work should be followed by appropriate stress relief or full solution treatment and aging to recover a uniform, reliable high-strength microstructure for demanding service.
