————————————————————————————————————————————————— Inappropriate coil design parameters and process recipe may result in crack development near oil holes. Longitudinal and transverse holes an induction heated component can cause an undesirable redistribution of eddy current flow, generating excessive heat in certain regions of the workpiece, which could potentially lead to localized overheating.
Overheating can cause unwanted metallurgical microstructures, excessive grain growth, scale, shape distortion, and decarburization, as well as grain boundary liquation, which weakens the grain structure in the steel and substantially increases brittleness and sensitivity to developing intergranular cracking upon quenching.
Hole size, location, and orientation can have a marked effect on eddy current flow and severity of overheating. In addition, other factors, such as steel chemical composition, can complement overheating, increasing crack sensitivity. Steels having higher carbon content are more prone to cracking. Other elements can affect crack sensitivity, depending on the amount and combination of elements present. Certain alloying elements are purposely added to provide specific properties such as strength and toughness, while other elements are contained in steel in trace amounts or as residual impurities of raw materials. An unfavorable combination of the latter elements can promote a tendency to cracking.
Using lower than optimal quench temperatures, higher than specified quench flow rates and pressures, and lower quenchant concentrations can also initiate hole cracking. These and other quench parameters should be evaluated if cracking suddenly appears.
Prevention of the oil-hole overheating and developing an optimal coil design is the first step in elimination of crack development. An equally important step is avoiding an undesirable distribution and magnitude of residual stresses, particularly transitional stresses. Predicting the formation of stresses during and after induction hardening requires expert modeling and simulation. For example, the animation below shows the results of FEA computer simulation of austenite transformation upon spray quenching around an oil hole. (The study was conducted by Deformation Control Technology Inc.)
(Courtesy of Deformation Control Technology Inc.)
Experience shows that in many cases, the proper choice of design parameters (applied frequency, power density, coil profiling, etc.) allows obtaining the required hardened pattern around holes free of cracks, even in those cases that at first might seem unsuitable for heat treating by induction.
• V. Rudnev, Computer modeling helps prevent failures of heat treated components, Advanced Materials & Processes, p 28-33, October 2011.
• V. Rudnev, D. Loveless, et al., Handbook of Induction Heating, Marcel Dekker, NY, 2003.