4140 vs 4340: the datasheets match, so why do people argue?
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TL;DR: At the same strength class the 4140 and 4340 numbers in our datasheets are the same. The real differences are hardenability in thick sections and the strength range the grade is sold in. Choose the condition, not just the grade name. The situation: someone says "use 4340, it is better than 4140", someone else says it is the same thing for half the price. I went to the datasheets in our archive to see who is right. At the same class they match Voestalpine's 4140 sheet lists two quenched and tempered classes: 665 MPa yield with 850 to 1000 MPa tensile, and 740 MPa yield with 930 to 1080 MPa tensile. Elongation is 12 %, impact energy 42 J, hardness 269 to 331 HB. The 4340 sheet uses lettered conditions, and T and U are the same two classes: 665 and 740 MPa yield, 850 to 1000 and 930 to 1080 MPa tensile, elongation 13 % and 12 %, impact energy 35 J and 42 J. Compare those lines and you cannot tell the steels apart. But the 4340 sheet keeps going Conditions V, W, X, Y and Z push yield through 835, 925, 1005, 1080 and 1125 MPa, with tensile up to 1550 MPa. And watch what happens to toughness: elongation falls from 13 % to 5 %, and impact energy falls from 42 J to 9 J. Same grade, completely different part. The sheet also marks the top conditions with an asterisk, noting such bars are typically ordered annealed for machining and then heat treated. So where do they really differ? 4340 has nickel on top of chromium and molybdenum, and nickel lets steel harden deeper, so the core of a thick section can match the surface. That is general metallurgy, not a number from our tables. In a thin bar the two behave alike. In a big shaft or forging, 4340 is the safer way to get uniform properties through the section. 4140 is the cheaper, easier-to-find workhorse for axles, gears, tooling and general machinery. How I would choose 1. Section size: thin or medium bars, 4140 is usually enough; thick sections with high strength demands, look at 4340. 2. Strength class: decide the yield and toughness you need, then find the condition that gives both. The highest condition is not the best one, as the impact numbers show. 3. Heat treatment: these values belong to defined quench and temper routes. If your heat treater cannot hold that route on your section size, the grade difference is academic. 4. Cost and supply: 4140 wins unless section size forces your hand. Comparing two steels at different conditions is how people convince themselves one grade is stronger. Compare the same condition and the same section. For people who machine or heat-treat these every day: at your sizes, do you actually see a difference, or is it mostly habit and whatever the supplier stocks?