Alloy structural steel has been widely used in machinery industry.Heatking analyzed the feature of alloy steel phase change as following: since alloystructural steel actually is adding one of several kinds of alloying elementsinto the carbon structural steel to improve quenching eligibility of steel, viaheat treatment to improve the overall strength and toughness. However inductionheating is mainly applied to surface or partial surface hardening of parts withhardening depth 1-3 mm, medium-carbon structural steel’s hardenability can meetsurface hardening requirement of most parts. For axle shaft and knuckle ofmedium and heavy vehicles the hardening depth requires normally over 4 mm dueto large load, but the hardenability of medium-carbon structural steel don’tguarantee such the hardening depth, so medium-alloy structural steel has to beused. The normal trade mark is: 40Cr and 4OMnB.

During induction heating alloying element has strongimpact on austenitic formation, austenite grain size and metallographicstructure after quenching, mainly shown on the following aspects:

①most of alloyingelements make the eutectoid steel point move in the direction of low carbonconcentration so as to change the original structure of steel;

② alloyingelement changed phase transformation starting temperature;

③alloying elementaffects the stability of the carbide;

④alloying elementaffects the carbon diffusion velocity in iron;

Now chromium, manganese, boron and other elements’ effectcan be analyzed the following:

Chromium is one of the carbide forming elements and itsexistence can lead to carbon’s diffusion velocity in iron decrease, its alloycarbide is stable which can make phase transformation process slow. So chromiumcan raise the phase transformation temperature obviously and widen the phasetransformation temperature range. For example, steel 40Cr has quenchingtemperature 900-960℃ during inductionheating.

Manganese is one of the no-carbide forming elements andits affinity with carbon is better than with iron. The stability of manganesecarbide is similar to cementite, so it has small impact on carbon’s diffusionvelocity. Manganese can reduce Ac1 point so it can get austenite withoutnegative capability for austenite grain growth when its heating temperature islower than critical point of carbon steel. Based on above reasons manganesesteel quenching temperature is the same as/lower than carbon steel with samecarbon content. For example, with same original structure and under sameheating speed steel 45Mn2 has quenching temperature 860-920℃ but steel 45# has 880-920℃.

Boron is one of the rich alloy elements in China. Addingmicro-amounts boron（0.0005%-0.005 %）into steel can improve the hardenability of steelsingnificantly, so it can replace many rich alloy elements. Promoting theapplication of boron steel has important meaning for China’s economy.

Boron steel 40MnB ,45MnB,etc. are often applied ininduction heating. Adding micro-amounts boron into steel has no obviousinfluence to steel’s phase transformation but it can make austenite grain grow.Some documents show that 4oMnB has critical point Ac1-- 730℃, Ac3--780℃ which is similar to steel 40#’s critical point, so their quenchingtemperature is similar. Steel 40MnB has induction quenching teperature 560-900 ℃.

Alloy elements like Ti, V, M form stable indissolvablecarbide in steel. While induction heating due to fast heating speed, no holdingtime, for alloy carbide it’s too late to dissolve. Austenite is low-carbonduring quenching cooling alloy carbide became transformed crystal nucleus, sothis reduces hardenability of steel. Raising quenching temperature or heatingtime can improve depth and hardness of hardened case but increases the tendencyto quenching crack. So alloy steel with elements Ti、V、Mo is rarely used ininduction hardening parts.