Engineering Guide · 3 min read
Choosing Carbon Steel under GB/T 699: 35#, 45# or 35Mn?
What do 35# and 45# mean, and how does 35Mn differ? Compare GB/T 699 carbon and manganese ranges and their effects on hardness and hardenability.
The short answer: 35# and 45# are GB/T 699 carbon steel grades; 35Mn has the carbon range of 35# with more manganese to improve hardenability. [1] [2]
For a forged pin or machined stud, carbon content helps determine the hardness available after heat treatment. Manganese offers a separate way to change hardening depth while retaining a selected carbon range. GB/T 699-2015 provides both choices. [1]
This article is part of the Chinese GB/T steel grades series.
35# and 45#: how the names are written
In Chinese trade usage, 35# means No. 35 steel and 45# means No. 45 steel. The standard grade names are 35 and 45. An English drawing or material description can identify them as 35 steel — GB/T 699-2015 and 45 steel — GB/T 699-2015.
The two digits represent approximate carbon content in hundredths of a percent: 35 centers on about 0.35% carbon, and 45 on about 0.45%. The permitted intervals are given in the table below.
Composition of 35 steel, 45 steel, 35Mn and 40Mn
| Grade | C (%) | Si (%) | Mn (%) |
|---|---|---|---|
| 20 | 0.17–0.23 | 0.17–0.37 | 0.35–0.65 |
| 30 | 0.27–0.34 | 0.17–0.37 | 0.50–0.80 |
| 35 / 35# | 0.32–0.39 | 0.17–0.37 | 0.50–0.80 |
| 40 | 0.37–0.44 | 0.17–0.37 | 0.50–0.80 |
| 45 / 45# | 0.42–0.50 | 0.17–0.37 | 0.50–0.80 |
| 35Mn | 0.32–0.39 | 0.17–0.37 | 0.70–1.00 |
| 40Mn | 0.37–0.44 | 0.17–0.37 | 0.70–1.00 |
| 45Mn | 0.42–0.50 | 0.17–0.37 | 0.70–1.00 |
Composition values are mass percentages. Selected heat-analysis ranges, Table 1. # is the Chinese trade spelling; the standard grade names are numerical. [1]
Other grades include 08, 10, 15, 25, 50 and 55, as well as 15Mn, 20Mn, 25Mn and 30Mn.
35 steel versus 45 steel: what more carbon changes
35 steel is a medium-carbon grade with C 0.32–0.39%. 45 steel raises that range to 0.42–0.50%. More carbon tends to increase the hardness of annealed and hardened steel under comparable processing. [2]
45 offers the higher-carbon starting point when a pin or forged part needs greater hardness after hardening. Under comparable annealed conditions, it is also harder than 35, which affects forming and machining.
GB/T 699 includes hot-rolled and forged bar, with heat-treated delivery available by agreement. Specify the grade together with the delivery condition so that the supplier knows both the required chemistry and the condition of the stock. [1]
35Mn and 40Mn: what manganese adds
35Mn has the same carbon range as 35, but a higher manganese range. The comparison is 0.70–1.00% Mn for 35Mn against 0.50–0.80% for 35. Similarly, 40Mn and 40 share C 0.37–0.44%, with the higher-manganese interval in 40Mn.
Manganese increases strength and hardenability: the ability of steel to harden through its section during quenching. [2] This is the reason to consider 35Mn when the carbon range of 35 is suitable but greater hardening depth is useful. The same comparison applies to 40 and 40Mn. The finished hardness still depends on the quenching and tempering conditions.
The gain over Q195 and Q235
The previous article, Q195, Q235 and Q275 steel under GB/T 700, describes structural strength families. GB/T 699 gives more deliberate choices of carbon interval and manganese content. That is the advantage for a forged or machined component whose final properties depend on its material and heat treatment.
In this standard, "quality carbon structural steel" names a defined material category. The grade and delivery condition provide the useful purchasing information.
Why cold-heading steel gets its own family
Numerical carbon steel defines a general structural material. When the manufacturing method is cold heading, the incoming stock also has to tolerate the intended deformation and meet the relevant surface and processing requirements. The ML08Al, ML10, ML15 and ML20 cold-heading family adds that forming purpose to the material specification.
ML35, ML40 and ML45 add cold-heading wire-rod requirements to a medium-carbon quench-and-temper route.
Browse the HexFastener homepage or submit a drawing with the required material callout.
References
- GB/T 699-2015 — Quality carbon structure steels. Table 1; numerical and manganese grade chemistry.
- Nippon Steel — Special Steel Strip. Printed pages 29–30; general effects of alloying and heat treatment.