Engineering Guide · 2 min read
Form First, Harden Later with ML35, ML40 and ML45 (GB/T 28906)
Why use ML35, ML40 or ML45 for cold-headed fasteners? Compare GB/T 28906 carbon ranges and the roles of annealing, quenching and tempering.
The short answer: ML35, ML40 and ML45 are medium-carbon cold-heading steels whose higher carbon ranges support hardening after forming, followed by tempering to adjust hardness and toughness. [1] [2]
GB/T 28906-2025 groups ML25, ML30, ML35, ML38, ML40, ML45 and ML50 in its carbon-steel quench-and-temper category. The selection spans several carbon intervals for stock that will be formed before final heat treatment. [1]
This article is part of the Chinese GB/T steel grades series.
ML35, ML40 and ML45: the carbon sequence
The ML prefix identifies cold-heading and cold-extrusion steel. The number retains the nominal carbon convention: ML35 centers on approximately 0.35% C, ML40 on 0.40%, and ML45 on 0.45%. The standard puts these grades in its quench-and-temper type.
ML38 lies between ML35 and ML40, with overlapping carbon intervals. ML25 and ML50 extend the same selection toward lower and higher carbon contents.
ML35 chemical composition and neighboring grades
| Grade | C (%) | Si (%) | Mn (%) |
|---|---|---|---|
| ML25 | 0.23–0.28 | 0.10–0.30 | 0.30–0.60 |
| ML30 | 0.28–0.33 | 0.10–0.30 | 0.60–0.90 |
| ML35 | 0.33–0.38 | 0.10–0.30 | 0.60–0.90 |
| ML38 | 0.36–0.41 | 0.10–0.30 | 0.60–0.90 |
| ML40 | 0.38–0.43 | 0.10–0.30 | 0.60–0.90 |
| ML45 | 0.43–0.48 | 0.10–0.30 | 0.60–0.90 |
| ML50 | 0.48–0.53 | 0.10–0.30 | 0.60–0.90 |
Composition values are mass percentages. Selected heat-analysis ranges, Table 3; its agreed low-silicon option has additional conditions in the standard. [1]
ML35 has C 0.33–0.38% and Mn 0.60–0.90% in this document. ML40 and ML45 raise carbon while keeping the manganese interval shown.
Why the manufacturing route includes quenching and tempering
Annealing can soften the stock and prepare its structure for heading. After the head is made, quenching hardens the part; tempering then adjusts hardness and toughness. [2] Separating these stages lets the manufacturer form relatively workable stock before developing the final hardened condition.
Compared with ML08Al, ML10 and ML20, the higher carbon intervals of ML35–ML45 increase the hardness attainable through hardening. The manufacturing tradeoff is stock preparation for forming plus a subsequent quench-and-temper operation.
ML35 alongside 35 steel
35 steel, often written 35#, has C 0.32–0.39%, Si 0.17–0.37% and Mn 0.50–0.80% in GB/T 699-2015. ML35 wire rod in GB/T 28906-2025 has C 0.33–0.38%, Si 0.10–0.30% and Mn 0.60–0.90%. Alongside those chemistry differences, the ML specification addresses the rod supplied for cold heading.
Manganese variants in the same processing category
The same standard includes ML15Mn, ML25Mn, ML40Mn2 and ML45Mn2 in the quench-and-temper category. For example, ML40Mn2 has C 0.38–0.43% and Mn 1.40–1.80%; ML40 has the same carbon range but Mn 0.60–0.90%. [1]
The added manganese increases hardenability while the nominal carbon level stays near 0.40%. [2]
What boron steel adds next
More carbon can raise attainable hardness under comparable processing. A small boron addition improves hardenability: the ability to develop a hardened structure through the section. The ML20B, ML20MnB and ML20MnTiB boron-steel family uses that approach at lower carbon levels.
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References
- GB/T 28906-2025 — Hot-rolled steel wire rod for cold heading. Official online text; Clause 4 and Tables 1–6.
- Nippon Steel — Special Steel Strip. Printed pages 29–30; general effects of alloying and heat treatment.