May 2021 Volume 3

FORGING RESEARCH

relatively small core dimension could not sustain a large tensile stress during the case phase transformation tomartensite. A thermo- mechanical coupled finite element model was built to simulate the entire carburizing heat-treating process. The phase transformation mechanisms and stress changes occurring in the part during quenching are described below.

a – Area of interest,

b – Temperatures at P1 and P2 over 10 cycles

c – Maximum principal stresses at P1 and P2 d – Effective stresses at P1 and P2 Figure 33. State variables in the upper die

Figure 35. Intergranular fractures in the core of the small rib area of a bearing cone From Figure 36, it can be seen that at the beginning of quench (0 to 2 sec), the surface of the part started cooling first and was prepared to contract, but the core was still at a high temperature, which prevented the surface from contracting and caused the tensile stress on the surface. At 2 to 3 sec, the core started cooling. Because of the lower temperature in the surface layer, the core experienced larger contraction than the surface, leading to the tensile stress in the core and the compressive stress in the surface layer. At about 280°C, the core initiated martensite transformation ahead of the surface because of its lower carbon content. From austenite to martensite, a volume expansion (4.64-0.53%C) was created. The expansion caused the compressive stress in the core and the tensile stress in the surface layer. The continuous cooling on the surface generated the compressive stress, which superimposed with the tensile stress. During the course of the temperature drop from 280°C to 84°C, martensite transformation moved from the core toward the surface, which pulled the core and pushed the surface. As a result, the core became tensile and the surface became compressive, so that the blue and red curves seen in the graph below gradually switched positions. At about 84°C, the surface started martensite transformation. The surface expansion was restrained by the already transformed core and hence the result was a compressive surface and a tensile core. Such tensile stress can trigger intergranular fracture if weak or brittle phases existed along the grain boundaries.

a – Area of interest

b – Temperatures at P1 and P2 over 10 cycle

c –Maximum principal stresses at P1 and P2 d – Effective stresses at P1 and P2 Figure 34. State variables in the center punch 6.5 Finite element modeling of thermal- and phase transformation-induced distortions In any heat treatment process, a steel component will experience size and shape distortions [7]. Size distortion is the expansion or contraction of the component because of the thermal effect and the phase transformations. Shape distortion is caused by non-uniform thermal and transformation stresses in a component with varying geometric sections and is also a result of the residual stress rebalance. These transient thermal-induced and phase transformation-induced stresses, as well as the residual stress, could be responsible for surface or inner cracking in the workpiece during treatment, depending on where the tensile stresses are located. In this example, intergranular fractures were observed in the core of the small rib area of a bearing cone after it was quenched in a carburizing heat treatment, as shown in Figure 35. Intergranular fracture is the propagation of cracks along the grain boundaries of a metal or alloy containing the weak or brittle phase. It can spread rapidly with little or no plastic deformation (embrittlement). In this case, it was assumed that the fractures happened when the

Figure 36. Hoop stress changes in the small rib area during quench

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FIA MAGAZINE | MAY 2021