1. The Two Families
Round bars for machining fall into plain medium-carbon steels, represented by SAE 1045, and chromium-molybdenum alloy steels, represented by 42CrMo to GB/T 3077, 4140 to ASTM A29 and SCM440 to JIS G4105. The plain grades are economical and respond to surface hardening, while the alloy grades offer deeper hardenability, higher strength after quenching and tempering, and better fatigue performance.
2. Chemistry and Hardenability
SAE 1045 contains about 0.45 percent carbon without deliberate alloy additions, so it hardens a relatively shallow layer. The chromium-molybdenum grades add roughly one percent chromium with about 0.2 percent molybdenum, which greatly increases the depth to which a section can be through-hardened. That difference is decisive for large shafts where the core must reach the required strength.
3. Heat Treatment and Properties
All four grades are normally supplied in a hot-rolled, forged or heat-treated condition and are then quenched and tempered by the parts maker. The chromium-molybdenum steels accept higher tempering temperatures and give a better balance of strength and toughness at the same hardness. Surface hardening by induction or nitriding is common for wear surfaces such as journals and splines.
4. Machining and Shaft Applications
SAE 1045 machines readily with coated carbide tooling and is used for shafts, axles, bolts and pins where cost matters. The alloy grades are chosen for crankshafts, gear blanks, connecting rods, heavy-duty shafts and tools that see cyclic loading or high torque. Forging improves the grain flow around the part, which is why forged bars are preferred for highly stressed components.
5. Ordering and Quality
Orders should state the grade and standard, diameter, length, delivery condition and any ultrasonic or magnetic particle testing requirement. Buyers should confirm the heat number and the certificate content, including chemistry, hardness and the results of any non-destructive examination. Size tolerances, straightness and the condition of the bar ends also affect machining yield.
Frequently Asked Questions
Q: What is the difference between 4140 and 1045?
4140 is a chromium-molybdenum alloy steel with much deeper hardenability, while 1045 is a plain medium-carbon steel.
Q: Are 42CrMo, 4140 and SCM440 the same?
They are closely equivalent grades from Chinese, American and Japanese standards respectively.
Q: Which grade suits large shafts?
A chromium-molybdenum grade, because a plain medium-carbon bar cannot be through-hardened in large sections.
Q: Can these bars be supplied forged?
Yes, forged round bars are common and improve grain flow for highly stressed parts.
Q: What tests can be specified?
Ultrasonic testing, magnetic particle testing, hardness and full chemistry on the heat.