The Quenched And Tempered Process And Surface Rolling Reinforcement Of42CrMo Yoke Bars

In most cases, the diameter of42CrMo steel made for long yoke bars is in dangerousquenching size. When quenching, the steel is to be out of shape or cracks easily.42CrMo steelwas heat treated by quenched and tempered，subcritical quenching and MF induction heattreatment. The microstructure and property of the steel were tested by metallographicmicroscope, hardness tester and the tensile test after heat treatment. The dangerous sizes of oilquenching and water quenching are Φ36mm~41mm and Φ45mm~50mm, respectively. If theoriginal structure is banded ferrite and pearlite, it is easy to crack when subcritical quenching.The strength and hardness of the forged steel that was treated with the subcritical quenching andhigh temperature tempering are close to the conventional quenched and tempered treatmentwhile plasticity and toughness are better. Through heating type medium frequency inductionheating heat treatment, the hardness along the radius of workpiece changed smally. Itsmechanical properties were superior to pallet car furnace heat treatment. After quenched andtempered using pallet car furnace heating, the grinding quantity was12.57kg/piece. It was12times as much as medium frequency heating quenched and tempered (1.096kg/piece).Traditional processing makes both the straightness along the axis of the yoke bars and thesurface quality poor. It is hardly to meet the requirements of modern large-scale equipment. Across-rolling was applied on the surface of42CrMo long yoke bars to strengthen the surfacelayer and ensure high dimension precision. A nanostructural layer was formed on the surface ofthe bars rolled by the cross-rolling. The layer microstructure was composed of nano-scale grainsand some deformation grains, which contained compressive residual stress of-479MPa. Thedepth of compressive stress layer is0.65mm or more. The surface roughness decreased fromRa2.1μm to Ra0.14μm. The surface micro cracks were repaired. The diameter tolerance washigher than H9. Ovality could be controlled within0.02mm. The whole property of yoke barsincreased slightly. The machining process was optimized.