Induction hardening is a form of heat treatment in which a metal part is heated by induction heating and then quenched. The quenched metal undergoes a martensitic transformation, increasing the hardness and brittleness of the part. Induction hardening is used to selectively harden areas of a part or assembly without affecting the properties of the part as a whole. The hardening depth is 1.5-5mm; medium/low-frequency quenching may lead to excessive depth and negative effect on strength; high-frequency quenching generally result in a shallow depth, and may ensure the balance of hardness and strength.
AISI SAE 4140 alloy steel is a chromium molybdenum alloy steel specification widely used in general purpose high tensile steel for components, like cylinder rod, shafts, bolts, gears and other applications.
Common 4140 Steel Specifications
| Country | USA | German | British | Japan | China | Australia |
| Standard | ASTM A29 | DIN 17200 | BS 970 | JIS G4105 | GB/T 3077 | AS 1444 |
| Grades | 4140 | 1.7225/ 42crmo4 | 42CrMo4 | SCM440 | 42CrMo | 4140 |
3. 4140 Steel Bar Chemical Composition
| Standard | Grade | C | Mn | P | S | Si | Ni | Cr | Mo |
| ASTM A29 | 4140 | 0.38-0.43 | 0.75-1.00 | 0.035 | 0.040 | 0.15-0.35 | – | 0.8-1.10 | 0.15-0.25 |
| EN 10250 | 42CrMo4/ 1.7225 | 0.38-0.45 | 0.6-0.9 | 0.035 | 0.035 | 0.4 | – | 0.9-1.2 | 0.15-0.30 |
| JIS G4105 | SCM440 | 0.38-0.43 | 0.60-0.85 | 0.03 | 0.03 | 0.15-0.35 | – | 0.9-1.2 | 0.15-0.30 |