Cold Heading SUS630 Instead of CNC: Turning a Difficult Material into a Near-Net Shape

1. A material that is awkward at both ends

SUS630 is one of the few genuine all-rounders among fastener and precision structural materials. It reaches a tensile strength in the 1,300 MPa class while holding corrosion resistance in most atmospheres and common media at roughly the level of 304. Aerospace, oil and gas, chemical plant, medical devices and high-end machinery all use it.

One qualification has to be attached: 17-4PH is slightly less corrosion resistant than 304 or 316 overall. In chloride-bearing, acidic or high-temperature aqueous media it pits, and it is susceptible to stress corrosion cracking. It cannot simply be substituted wherever 304 or 316 was specified — selection has to be assessed against the actual medium, concentration and service temperature.

It is also one of the most frustrating materials in a machining shop, because both conditions are awkward:

• Machining in the aged condition: high strength, sticky cutting, rapid work hardening, severe tool wear, low cutting speeds and poor surface finish.
• Machining in the soft solution-treated condition: cutting is easy, but ageing still has to follow, and ageing changes the dimensions. Precision features must be re-machined after ageing, which means an extra clamping, an extra operation and an extra chance of going out of tolerance.

So the conventional route fossilises into a long and expensive chain: buy bar stock → rough turn in the soft state → age → finish turn or grind → inspect. The bar is bought at the largest diameter, chips come off by the barrow-load, and SUS630 is not a cheap material to begin with. Money leaks out in three places at once: metal bought and then cut away, inefficient cutting hours, and the second clamping after ageing.

That chain can be rewritten: solution-treated wire → multi-station cold heading near-net → rolled thread → age → minimal finish machining. This article explains why that works, where it is difficult, and how much it saves.

2. What SUS630 actually is

SUS630 carries different designations in different systems, and engineering communication frequently goes wrong on the assumption that same name means same material, or that different names mean different materials. Pin it down first:

It is a martensitic precipitation-hardening stainless steel, and neither word can be dropped. Martensitic means it is magnetic and can be substantially strengthened by heat treatment, unlike austenitic 304 or 316 which can only be strengthened by cold work. Precipitation hardening means the strengthening comes from a low-temperature ageing precipitate rather than from quenching to high-carbon martensite, so high strength is achieved with a comparatively controlled loss of toughness.

Typical chemical composition, per GB/T 1220 and ASTM A564:

Strengthening happens in two steps, and understanding them is the precondition for understanding why cold heading is possible:

• Step one, solution treatment: heat to about 1,040 °C, then air or water cool, holding copper and niobium in solution to produce a low-carbon martensite. This is Condition A, the softest, most ductile and most formable state — and the only window in which cold heading can happen.
• Step two, ageing: hold between 480 °C and 620 °C so that copper-rich and niobium-bearing phases precipitate and dispersion-strengthen the matrix. Strength and hardness rise sharply. The lower the ageing temperature, the higher the strength and the lower the toughness.

Placed against the stainless fastener family tree, the position becomes clear:

The selection logic in one sentence: when corrosion resistance and high strength must be satisfied at the same time, and the SCC risk of martensitic stainless is unacceptable, SUS630 is one of the few answers.

3. Ageing temperature decides everything

Choosing SUS630 is not one decision but two: the grade, and then the ageing condition. The same grade delivers completely different strength and toughness combinations depending on ageing temperature. The values below are typical references; the product standard or the supplier's measured data governs.

Three engineering rules stand out from that table:

• Strength and toughness trade against each other. From H900 to H1150, tensile strength falls from 1,310 to 930 MPa, about 29 %, while impact toughness improves markedly. For parts under impact, vibration or low temperature, chasing H900 is often simply wrong.
• Strength and corrosion resistance also trade. In the high-strength conditions (H900, H925) both corrosion resistance and stress corrosion resistance drop, and hydrogen embrittlement sensitivity rises. This is why ASTM F593 and similar standards bind ageing condition to strength class and restrict where the high-strength conditions may be used.
• Ageing shrinks the part. Copper-rich precipitation is accompanied by a dimensional contraction of roughly 0.04 % to 0.06 %, or 0.0004 to 0.0006 mm/mm. On an M6 thread that is micrometres — trivial. On a precision fit, a coaxiality callout or an overall length, it is enough to scrap the part.

All three rules point to one conclusion: the manufacturing sequence for a SUS630 part must be form first, age second, finish machine third. And if the forming step can move from cutting to cold heading, the finishing allowance in that third step shrinks dramatically. That is precisely the gap cold heading fits into.

4. Why the CNC route costs so much

Leak one: the material account

Turning has to start from bar larger than the part's largest diameter, while cold heading starts from wire close to the effective section. On low-carbon steel that difference is an expense. On SUS630 it can be the variable that decides whether the project is viable at all, because stainless bar carries a high purchase price and turning produces more than 60 % of it as chips.

A classic comparison from multi-station cold heading practice: the same part needs 3,000 lb of bar stock per 1,000 pieces by turning, and 326 lb of wire by cold heading. A saving of 2,674 lb, close to 89 %. For austenitic and martensitic stainless and alloy steel — materials where the raw material dominates part cost — cold heading can cut the material bill by an order of magnitude.

Leak two: the hours account

SUS630 cuts slowly in the aged condition and tool life is short; that is unavoidable. The bigger problem is the two clampings: rough machine soft, age, finish machine. Clamping is not free — it brings tool-setting time, re-alignment error, and the extra stock that has to be left on because ageing will move the part.

Leak three: the quality account

Turning severs the grain flow, and the typical failure location on a SUS630 part is exactly the fillet between head and shank. Every additional machining pass is another chance to leave a tool mark or a scratch, and those marks are natural initiation sites for fatigue cracks.

Leak four: the hidden account — the 85 %

On the familiar 15/85 rule for total cost of ownership, the visible direct purchase price is only about 15 % of what a joint actually costs; the other 85 % hides in design selection, supply chain, assembly, inspection and after-sales. A pure CNC route leaves its own liabilities there: quotation instability as tool prices move, batch consistency that depends on operator skill, long lead times that force stock holding, and the hydrogen embrittlement and delayed fracture risk of high-strength parts.

5. Why cold heading suits SUS630 — and the five challenges

Feasibility: 17-4PH is already on the cold-forming list

This is not an adventure. In the material capability lists published by the major cold-forming machine builders, 17-4PH is explicitly named as a cold-formable stainless, alongside 302HQ, 316L, 321, 405, 410, 420, 430F, 431 and A286. Those lists describe stainless as offering excellent corrosion resistance, work hardening through cold forming — particularly important for grades that cannot be strengthened by heat treatment — good heat resistance, and magnetic or non-magnetic options depending on grade.

17-4PH is further annotated with a dedicated cold-forming process name (Camtronic), which shows the industry has already developed a multi-station route specifically for it. The question is not whether SUS630 can be cold headed, it is how to get the process parameters right.

There is one piece of good physical news: SUS630 has very low carbon, 0.07 % maximum. Cold heading practice repeats one principle over and over — the higher the carbon content, the worse the cold formability and the lower the permissible deformation. The low carbon of a low-carbon martensitic matrix is exactly what allows it to be formed cold in the solution-treated state.

Challenge one: high work hardening — take the deformation in steps

Stainless work-hardens far more than low-carbon steel. Cold heading is itself a cold-working process: the larger the deformation q, the more ductility is consumed and the harder further deformation becomes, until the part cracks. The industry rule of thumb: for products with 65–85 % deformation, formed in at most three blows without intermediate heat treatment, carbon content must not exceed 0.5 %; above that, heating is required. SUS630 sits well inside that limit, but chromium and nickel raise the yield point and reduce ductility at the same time, so the practical window has to be spread across several stations — each blow completing only part of the total deformation.

The number of blows is not a matter of feel; it follows the ratio of blank free height to wire diameter, h/d_m: up to 2.5 in one blow, up to 4.5 needs two, up to 6.5 needs three, and beyond 8 the work has to be split across two machines with a heat treatment in between. For a high-flow-stress material like SUS630, multi-station forming with four, five or six stations is effectively standard — a modern six-station former takes less than half a second from wire to finished blank.

Challenge two: high flow stress — recalculate tooling and press tonnage

Cold heading force is proportional to the strength of the metal being upset. Stainless has the highest deformation resistance of the common cold-heading materials. A useful set of relative resistance coefficients: copper about 0.6, brass 0.9, grade 15 steel 1.1, grade 25 1.2, grade 35 1.3, grade 45 1.4, grade 60 1.5–1.6. Stainless, with its alloy content and strong work-hardening tendency, sits above carbon steel of the same strength level. The consequences are a larger press, higher contact stress on the tooling, and tool life becoming one of the decisive variables in unit cost. The countermeasures are cemented carbide inserts such as the BK8 class, and managing tool life as a monitored production parameter rather than a reaction to failure.

As an order of magnitude: die bore diameter grows with output by roughly 0.05–0.10 mm on small bores, 0.08–0.12 mm on medium bores and 0.10–0.15 mm on larger bores around 20–24 mm, and the wear becomes uneven and elliptical. Tool life planning therefore means changing dies on a wear curve, not waiting for a failure.

Challenge three: friction and galling — the hidden threshold

This is the most commonly underestimated part of stainless cold heading. Under high pressure stainless adheres to the die, tears and galls, which ruptures the surface and destroys the tooling quickly. The classic answer is a complete surface preparation sequence, of which the most characteristic step is depositing a thin copper layer on the stainless wire before heading:

The value of the copper route is that it establishes a film between die and blank that shears predictably, maintaining uniform sliding even at extreme pressure, which lowers the ejection force and greatly extends ejector pin life. Related approaches include graphite die lubrication — handbook friction coefficients run about 0.05–0.10 for a polished surface with graphite, 0.10–0.15 for a polished surface without, 0.15–0.20 for a finish-machined tool and blank face, and 0.20–0.30 for a rough-machined face — plus phosphate and soap coating on the wire.

A note on lubricant choice: sulphurised cutting oils are common, with sulphur additions up to 10 % for carbon steel and 20 % for alloy steel. They lubricate effectively but are harmful to operators' skin and require an extra cleaning operation afterwards. For stainless, copper plus graphite is usually better on both process stability and downstream cleanliness.

Challenge four: precipitation-hardening grades are extremely sensitive to surface defects

Cold heading stock must be clean and free of scale marks. Scale spalls off when the blank is sheared, lodges between the cutter and the die, and works into slideways and punch carriers where it grinds moving parts and jams them. More importantly, porosity, laps, seams, cracks and burrs open up under high-pressure cold heading and cause fracture. For a high-strength material like SUS630 the requirement is stricter than for low-carbon steel.

The incoming wire gate therefore has to be confirmed up front: no scale, no porosity, no laps, no seams, no cracks, no burrs, and no rough draw marks, streaks, scars, laminations or scuffing on the surface. For reference, the general technical requirement for cold-heading annealed steel wire is tensile strength not above about 70 kgf/mm², elongation not below 12 %, reduction of area not below 50 %, and a pass in an upsetting test at the specified deformation with no cracks or splits in the specimen.

Challenge five: ageing shrinkage and the allowance split

Cold heading produces a solution-treated blank, and the dimensions move again during ageing. Cold work also leaves a small elastic springback. Process design must therefore settle three questions:

• Which dimensions come out of cold heading at final size — non-critical and cosmetic features.
• Which dimensions get a finishing allowance and are machined after ageing — fits, coaxiality, critical lengths.
• Which dimensions have the ageing shrinkage built into the cold heading size, for cases that cannot have a second operation but do have a tolerance.

Get this right and the cold heading plus ageing route saves operations without going out of tolerance. Get it wrong and the savings come back as rework.

6. Quantified benefits

Laying the benefits out line by line is more convincing than a headline claim of "40 % cheaper".

Rolled threads deserve separate emphasis. A well-known case in the trade: an internal-thread fastener in a motorcycle wheel assembly was originally a cold-headed blank with cut and tapped threads, and the cut internal thread could not meet the requirement of the automated assembly line, producing high rework. Switching to a cold-headed blank with rolled internal threads raised thread quality and cut assembly-line rework caused by thread problems significantly.

Another case shows the geometric freedom better. A tube plug body for an automotive engine assembly needed a taper thread to seat an O-ring, a cup base flat within 0.05 in, and a smooth first step inside the cup. A taper thread is difficult to turn. The final route was cold heading plus thread rolling, with a second operation meeting the tight bore tolerance, and material savings on top.

One more benefit is easily overlooked: turning an assembly into a single part. The classic example is a commercial vehicle air-conditioning valve stem, originally two 302 stainless parts turned and then brazed, with the brazed joint failing to meet strength requirements. It became a single cold-headed 3/8 in × 3 in part with a drive recess and square head, then turned, burnished and ground to 32 μin and the required coaxiality. The part is stronger, the brazing operation is gone, and both material and part cost fell.

7. Design rules for SUS630 cold-formed parts

Suitability criteria

A part that meets any one of the following deserves a cold heading feasibility review. Meet several and cold heading is very likely the better route.

Size and machine capability

• Diameter: about 1.27 mm to 40.6 mm cut-off diameter (0.020 in to 1.6 in).
• Length: under-head length up to about 355 mm at diameters to 38 mm, or about 432 mm at diameters to 22 mm, depending on shape complexity.
• Stations: modern formers offer six or more forming stations; a two-stage three-blow process reaches about 4.5 diameters of upset, larger head ratios and more varied shapes.
• Secondary machining tolerance: high-end secondary equipment performs up to 12 operations (turning, grooving, drilling) on one machine while holding about 0.025 mm on diameter and 0.127 mm on length.

Tolerance and allowance strategy

Heat treatment and surface finishing, including hydrogen embrittlement

Heat treatment options after cold heading include through hardening and tempering, case hardening, annealing, selective induction hardening and carbonitriding. For SUS630 the main line is ageing; the others serve specific functional needs.

Coating choice has to balance corrosion resistance, lubricity, appearance, cost and toxicity. One risk cannot be skipped:

• Hydrogen embrittlement. The higher the tensile strength, the greater the susceptibility. The combination of H900 or H925 with electroplating is therefore a high-risk configuration. Where high strength and plating are both required, prefer an intermediate condition such as H1025 or H1075, or switch to a non-electrolytic finish. Verification can follow GB/T 3098.17, the preload test on parallel bearing faces for hydrogen embrittlement.
• Corrosion ranking. SUS630 is more corrosion resistant than martensitic chromium steels such as 410 and 420, which are clearly below austenitic chromium-nickel steels and carry SCC risk, but it should not be used at 304 or 316 ratings — in chlorides it can still suffer dangerous internal stress corrosion and pitting. Verify against the actual medium rather than inferring from the material family.

8. When to stay with CNC

A proposal is only credible if its boundary is stated. Keep CNC for:

• Very small batches and one-off prototypes — the tooling front-load cannot be amortised.
• Complex internal cavities, cross holes and deep grooves — outside the cold-forming envelope.
• Fits at ±0.0025 mm — that needs ground bar stock at roughly three times the cost of standard bar.
• Development phases with frequent design changes — 6–12 h changeover and high tooling investment do not suit rapid iteration.

Even in those cases the optimum is rarely pure CNC. It is usually cold heading near-net plus a small amount of CNC on the critical fits. That is exactly the pattern used in the 6061 aluminium lifting sleeve case: cold form the base shape — fast, repeatable, material utilisation near 98 % — then a few quick CNC operations to turn the top thread and finish the seat face, taking unit cost from about CNY 50 to about CNY 20 and cycle time from 20 minutes to under 4, with higher resistance to stress fracture from the continuous grain flow.

9. Implementation roadmap

10. Conclusion: turn chips into flow lines

SUS630 is difficult to machine for one structural reason: it carries high strength and high corrosion resistance in the same material, and conventional machining can only choose between soft and formable but dimensionally unstable, or hard and stable but barely cuttable.

Cold heading offers a third path: press the shape once in the solution-treated state, take the strength once in the aged state, and then machine only the faces that genuinely need it. The material no longer leaves as chips; it stays in the part as continuous grain flow, which is where it belonged all along.

If you are producing a SUS630 fastener, pin or structural part on a bar-stock, turn, age, finish-turn route, it is worth a design-for-manufacture review. Sometimes the largest saving comes from changing the way the chips are made.

11. Sources and standards

• Cold Formed Specials — international cold-forming technical reference: cold-formable material list including 17-4PH, performance and cost benefits of cold forming versus turning, multi-station capability and size ranges, rolled threads, shapes that cannot be turned, secondary operations and heat treatment, coating selection.
• Metal Cold Heading Technology (B. M. Misuzhnikov and M. Ya. Grinberg, translated by the First Ministry of Machine Building General Machinery Research Institute): work hardening and deformation limits, carbon content versus cold formability, blow-count rules, heading force and material coefficients, copper plating of stainless before heading, die wear allowances, incoming wire requirements and the upsetting test.
• Cold Forming Basics — process principles and part suitability criteria.
• Fastener Technology Handbook — stainless fastener groups and corrosion resistance, martensitic chromium steel versus austenitic chromium-nickel steel and SCC risk, elastic modulus and thermal expansion comparisons.
• How Cold Heading Cut EV Battery Pack Lifting Sleeve Cost by 60 % and Made It Stronger — cost, cycle time and grain-flow benefits of the hybrid cold forming plus CNC route.
• Standards: GB/T 1220 (stainless steel bar), ASTM A564 / A693 (precipitation hardening stainless bar and plate), ASTM F593 (stainless steel bolts), GB/T 3098.6-2023 (mechanical properties of stainless steel bolts, screws and studs, including heat treatment of martensitic grades), GB/T 3098.25-2020 (selection guide for stainless and nickel alloy fasteners), GB/T 3098.17 (preload test on parallel bearing faces for hydrogen embrittlement).

Note: the chemical composition and aged mechanical properties quoted for SUS630 / 17-4PH are typical reference values, intended for process comparison and feasibility assessment. Formal design selection must follow the product standard (ASTM F593, ASTM A564) and the supplier's measured data.