Alloy steel grades
1 alloy steel grades from 1 designation systems, each with a full data sheet. Sorted by system; the yield and tensile values are the first line of each grade's mechanical table.
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AISI / SAE
| Grade | No. | Type | C % | Yield MPa | Tensile MPa | Equivalents |
|---|---|---|---|---|---|---|
| 4130 | G41300 | Chromium-molybdenum low-alloy steel, weldable (chromoly) | 0.28 – 0.33 | ≈ 360 | ≈ 560 | 25CrMo4, 30CrMo4, SCM430 |
Alloying buys depth, not surface hardness
Carbon decides how hard a steel *can* get; alloying decides how *deep* that hardness reaches. A 100 mm shaft in plain C45 hardens at the surface and stays soft in the core; the same shaft in 42CrMo4 hardens through. That is hardenability, and it is the reason alloy steels exist.
Chromium, molybdenum and nickel each do something specific: chromium raises hardenability and wear resistance, molybdenum suppresses temper embrittlement and holds strength at temperature, nickel adds toughness at low temperature. The EN name spells the recipe out — 42CrMo4 is 0.42 % carbon with chromium and molybdenum.
Reading an EN alloy designation
The convention is arithmetic. The first number is carbon in hundredths of a per cent. The letters are the main alloying elements in order of quantity. The numbers after them are the content of the first element multiplied by a factor — 4 for Cr, Co, Mn, Ni, Si and W, 10 for Al, Cu, Mo, Ti and V.
So 42CrMo4 is 0.42 % C and 4 ÷ 4 = 1 % chromium. 25CrMo4 is the same alloy at lower carbon. 34CrNiMo6 is 0.34 % C with 6 ÷ 4 = 1.5 % chromium plus nickel and molybdenum. Once the factor is known the designation stops being a code and becomes a specification.
Where alloy steel is the wrong answer
Alloy grades cost more, weld harder and need heat treatment to be worth anything. A structure that is welded and never heat-treated gains nothing from 42CrMo4 over S355 — the heat-affected zone will be the weak point either way.
The honest test: if the part will be quenched and tempered after machining, and if section thickness means a plain carbon grade would not harden through, an alloy grade is right. Otherwise it is an expensive way to buy the same yield strength.
faq.h
What does 42CrMo4 mean?
0.42 % carbon, with chromium as the main alloying element at 4 ÷ 4 = 1 %, plus molybdenum. In EN designations the divisor is 4 for Cr, Mn, Ni, Si, Co and W, and 10 for Mo, V, Ti, Al and Cu.
What is the AISI equivalent of 42CrMo4?
AISI 4140 is the usual near-equivalent, and 4142 is closer on carbon. They are not identical: EN 42CrMo4 has tighter limits on phosphorus, sulphur and residuals, and the EN standard specifies the delivery condition.
Why use alloy steel instead of more carbon?
Because carbon sets attainable hardness while alloying sets hardenability — how deep that hardness penetrates. A thick section in plain carbon steel stays soft in the core no matter how much carbon it has.
Can alloy steels be welded?
With preparation. Preheat, controlled interpass temperature, low-hydrogen consumables and usually post-weld heat treatment. Welding a quenched-and-tempered alloy steel destroys the heat treatment locally, so the joint design has to account for it.
What does the +QT suffix mean?
Quenched and tempered — the steel was austenitised, quenched and then tempered to a specified strength band. The same grade in +N (normalised) or as-rolled condition has quite different mechanical properties.