SCM435 vs SCM440: one alloy step apart, and it shows
Both are chromium–molybdenum steels from the same family. This page explains what the roughly 0.05% difference in carbon actually changes on a real fastener.
SCM435 vs SCM440
Short answer: SCM435 is the standard cold-heading grade for high classes; SCM440 is the answer when the section gets heavier or wear is in the requirement
SCM435 and SCM440 carry essentially the same chromium (about 0.90–1.20%) and molybdenum (about 0.15–0.30%). What differs is carbon: roughly 0.33–0.38% against roughly 0.38–0.43%. Everything else follows from that. SCM440 reaches a higher as-quenched hardness (typically around 55–58 HRC against around 53–56 HRC), keeps more hardenability margin in a heavier section, and resists wear better — at the cost of cold formability and quench-crack tolerance. Both grades genuinely reach class 10.9 and 12.9. In the diameters normal for cold-formed fasteners SCM435 is the default answer; SCM440 becomes the answer when the section grows — stud bolts, anchor bolts, larger bolts, thicker pins — or when wear as well as strength is specified.
SCM435 versus SCM440
| Decision factor | SCM435 | SCM440 |
|---|---|---|
| Near equivalents | Close to 34CrMo4 / AISI 4135 / 35CrMo | Close to 42CrMo4 / AISI 4140 / 42CrMo |
| Typical carbon | About 0.33–0.38% | About 0.38–0.43% |
| Chromium / molybdenum | Cr about 0.90–1.20%, Mo about 0.15–0.30% | Essentially the same |
| What actually differs | The same Cr–Mo alloy with roughly 0.05% less carbon | Roughly 0.05% more carbon; every other difference follows from it |
| Attainable as-quenched hardness | Typically around 53–56 HRC — the ceiling comes from carbon | Typically around 55–58 HRC |
| Hardenability by section | Through-hardens comfortably in cold-formed fastener diameters; margin narrows as the section grows | More margin in heavier sections — larger bolts, stud bolts, anchor bolts |
| Property classes realistically reached | 10.9 and 12.9 — the standard grade for socket screws and high-tensile bolts | 10.9 and 12.9 — chosen when section or wear demands it |
| Tempering to the same hardness | Tempers at a lower temperature | Needs a higher temper for the same hardness, which is useful headroom above the class minimum |
| Cold heading | Better — the reason it dominates socket screws and high-tensile bolts | Harder to cold head; often hot forged or machined instead |
| Quench-crack sensitivity | More forgiving of corner radii and cross holes | Less forgiving — geometry that survives on SCM435 can crack here |
| Wear resistance | Adequate for fastener duty | Better where a pin or thread flank sees wear |
| After zinc plating | At class 10.9/12.9 it is above the ISO 4042 trigger — bake | The same, and the higher the delivered strength the longer the bake the specification calls for |
How to choose
SCM435
Choose SCM435 for cold-formed parts that have to reach class 10.9 or 12.9 in normal fastener diameters — socket head cap screws, hex bolts, flange bolts. Its strengths are formability and tolerance of geometry, which usually means less scrap at the hardening stage for the same delivered quality.
SCM440
Choose SCM440 when the section is heavy enough that core hardness is in doubt — stud bolts, anchor bolts, larger diameter bolts, thicker pins — or when wear is part of the requirement alongside strength. Expect harder cold forming, and expect to control corner radii, cross holes and thread roots more tightly so the parts survive the quench.
Limits before putting it on the drawing
- These two are one alloy step apart, not two families. On many parts either will meet the class, and the real deciders are cost, forming route and crack risk — not the headline hardness number.
- The HRC ranges and composition percentages here are typical guidance; the binding figures are the material specification and the lot’s mill certificate, verified by test.
- The size at which a part still "through-hardens" depends on quenchant, agitation and geometry. As general guidance for this family in an oil quench, margin starts to tighten once diameters run past the mid-20 mm range — confirm with a sectioned part and a surface-to-core hardness traverse.
- Swapping SCM435 for SCM440 on paper is not a drop-in substitution: cold heading, corner radii and the quench conditions all have to be re-reviewed.
- Neither grade reduces the ISO 4042 obligation. Electroplated at class 10.9 or 12.9, both need de-embrittlement baking.
- If the drawing only needs class 8.8, either alloy is over-buying and importing an embrittlement risk you did not need — a boron cold-heading grade does 8.8 more cheaply.
- We heat treat and finish customer-supplied parts. We do not melt steel or form bolt heads.
What to send the heat treater
- 1The actual grade with its mill certificate — the real carbon figure drives the result
- 2Target property class or hardness range, with test location and method
- 3Diameter and thickest section
- 4Forming route (cold headed, hot forged, machined) and any history of cracking
- 5Downstream finish and the de-embrittlement requirement
- 6Lot quantity or weight, and sample pieces for a sectioned hardness traverse
Frequently asked questions
Is SCM440 simply stronger than SCM435?
Not in the way the question implies. At the same property class — say 10.9 — both have to meet the same minimum values. The real differences are that SCM440 has a higher hardness ceiling and more hardenability margin in a heavy section, while SCM435 forms more easily and cracks less readily.
Can SCM435 reach class 12.9?
Yes, and it is the standard grade for 12.9 socket head cap screws in normal cold-formed sizes. The things to watch are the minimum tempering temperature the class carries and de-embrittlement baking after electroplating.
Why did the SCM440 lot crack in the quench when identical SCM435 parts did not?
The extra carbon leaves a more highly stressed as-quenched structure, so the same geometry becomes a crack initiator. Sharp internal corners, cross holes near a thread root and abrupt section changes are the usual sites. Send the drawing with the crack location marked — it is normally visible in the geometry.