35CrMo and ML35CrMo: the difference is more than the prefix “ML”
When selecting steel for high-strength bolts, studs and specially shaped cold-headed parts, customers often ask whether 35CrMo and ML35CrMo can be substituted because their chemical compositions look similar. The grade name and chemistry alone cannot answer that question. The applicable product standard, supplied product form, composition limits, delivery condition and suitability for the intended cold-heading process must all be considered.
I. Different product-standard contexts
35CrMo is generally considered within the GB/T 3077 alloy structural steel system. It is a chromium-molybdenum steel commonly used in the quenched-and-tempered condition. Suitable heat treatment can provide a useful combination of strength and other mechanical properties. Applications include shafts, gears, connecting parts, fasteners and other load-bearing components.
The “ML” prefix in ML35CrMo identifies a different product context: steel for cold heading and cold extruding, commonly specified under GB/T 6478. For hot-rolled wire rod, GB/T 28906-2025 must also be considered as the applicable product standard. ML35CrMo is therefore not simply another spelling of 35CrMo. Material selection must address the intended cold-forming route.
II. Chemical composition in the three referenced standards
Composition values below are mass fractions, expressed as %. The comparison follows the stated editions and has been checked against their original tables. The P/S cells are controlled by steel quality class as explained below. The complete standard, inspection certificate and order specification still govern acceptance.
| Standard | Grade | C | Si | Mn | P | S | Cr | Mo |
|---|---|---|---|---|---|---|---|---|
| GB/T 3077-2015 Alloy structural steels |
35CrMo | 0.32-0.40 | 0.17-0.37 | 0.40-0.70 | By quality class; see note | By quality class; see note | 0.80-1.10 | 0.15-0.25 |
| GB/T 6478-2015 Steels for cold heading and cold extruding |
ML35CrMo | 0.33-0.38 | 0.10-0.30 | 0.60-0.90 | ≤0.025 | ≤0.025 | 0.90-1.20 | 0.15-0.30 |
| GB/T 28906-2025 Hot-rolled wire rod for cold heading |
ML35CrMo | 0.33-0.38 | 0.10-0.30 | 0.60-0.90 | ≤0.025 | ≤0.020 | 0.90-1.20 | 0.15-0.25 |
GB/T 3077-2015 Table 2 specifies P and S limits by steel quality class, in mass percent: quality steel, P≤0.030 and S≤0.030; high-quality steel, P≤0.020 and S≤0.020; special-quality steel, P≤0.020 and S≤0.010. State the applicable quality class in the order and inspection documentation.
These are selected composition limits from the stated editions, not a complete purchasing specification. Under GB/T 28906-2025 Table 3 note b, the lower Si limit may be below 0.10% by agreement; in that case aluminium must be added and Als≥0.020%. Note a permits grain-refining titanium up to 0.060%. Apply the complete standard and agreed conditions; do not infer material interchangeability from this comparison.
III. Why these differences matter in manufacturing
- Carbon: the source compares 0.32–0.40 for 35CrMo with 0.33–0.38 for ML35CrMo. A narrower carbon range is relevant to consistency after quenching and tempering and to controlling batch-to-batch strength variation.
- Silicon: the compared ranges are 0.17–0.37 and 0.10–0.30. Both ranges are 0.20 percentage points wide; the ML35CrMo range is lower, not narrower. Silicon control is one factor affecting plastic deformation, deformation resistance and cracking risk. Chemistry alone does not prove cold-heading suitability.
- Manganese: the source compares 0.40–0.70 with 0.60–0.90. The higher ML35CrMo range is relevant to hardenability and the subsequent heat-treatment response.
- Chromium: the compared ranges are 0.80–1.10 and 0.90–1.20. Chromium contributes to hardenability and the strength obtained after quenching and tempering; the applicable product requirements still govern acceptance.
- Molybdenum: the source gives 0.15–0.25 in GB/T 3077, 0.15–0.30 in GB/T 6478, and 0.15–0.25 in GB/T 28906-2025. The windows differ, so procurement must state the standard as well as the grade.
- Sulfur: the source gives a maximum of 0.025 for ML35CrMo in GB/T 6478 and 0.020 in GB/T 28906-2025. Sulfur and inclusion control are relevant to cold-heading cracking risk, particularly for multi-station operations and parts undergoing substantial deformation. The limit alone does not establish inclusion morphology or forming performance.
IV. Similar composition does not establish interchangeability
Both grades are chromium-molybdenum steels, but their product-standard contexts and manufacturing requirements differ. For cold-heading material, surface quality, cleanliness, microstructure, drawing behavior and forming consistency must be evaluated alongside chemistry and heat-treatment properties.
A compliant chemical analysis does not by itself prevent head cracks, flange cracks, accelerated tool wear or unstable production. The cause may involve delivery condition and suitability for the actual forming operation. Interchangeability requires engineering confirmation; it cannot be inferred from similar grade names.
V. What to confirm when purchasing
- Does the customer require 35CrMo or explicitly ML35CrMo?
- Which product standard and edition apply: GB/T 3077, GB/T 6478 or GB/T 28906?
- Is the purchased product round bar, wire or hot-rolled wire rod?
- Does the manufacturing route include drawing, spheroidizing annealing, cold heading, or quenching and tempering?
- Which fastener property class is required, for example 8.8, 10.9 or 12.9?
- Does the part involve substantial deformation, a head, a flange or other complex geometry?
- What requirements apply to decarburization, inclusions, surface defects and batch consistency?
VI. Practical conclusion
35CrMo and ML35CrMo should be assessed in their respective product and manufacturing contexts. For a cold-heading production route, the ML35CrMo product specification may be the relevant choice, but neither the grade name nor the fastener property class establishes suitability by itself.
Creation Group considers the customer's drawing, dimensions, deformation requirements, required mechanical properties, heat-treatment route, purchased product form and production history when evaluating these materials. The aim is a verified match between material and process, rather than substitution based only on chemical similarity.