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Heat Checking is a very important problem limiting the service life of aluminum die casting dies. In this paper, the relationship between the surface residual stress, which varies with die casting operation, and the initiation and propagation of heat checking is described.Measurement of residual stress and observation of heat checking were carried out on a cover and an ejector production die made by SKD61 (AISI H13). The surface residual stress was measured by X-ray and the heat checking was observed in detail by SEM after every prescribed shots from the beginning to the end of the die service life. The mechanism of residual stress variation and the effects of the position and the cavity shape of dies on the residual stress variation and the initiation and propagation of heat checking were clarified.The following conclusions were drawn from this investigation.(1) The surface residual stress, which was compressive before die casting operation, changed immediately toward tensile side and saturated in the early stage of operation, say at only 50 shots. This fact is considered to result from the following process; the surface layer of dies is rapidly heated by molten aluminum. The thermal expansion due to this rapid heating is restrainted by the substratum and thus the surface layer is subjected plastically to compressive strain. After cooling, the surface residual stress changes toward tensile side and saturates at some tensile value in the early stage of die casting operation.(2) The value of saturated tensile residual stress depended on the position and the shape of die cavity. This experimental result is caused by the magnitude of thermal load and restraint.(3) On the gate side of die cavity, in which the saturated tensile residual stress is large because of large thermal load, microcracks initiated in the saturated period. But on the position apart from the gate, in which the saturated tensile residual stress is small because of small thermal load, the residual stress did not decrease and microcracks did not initiate even at the end of die service life.(4) In the later stage of crack propagation on the gate side of die cavity, heat checking was observed by the naked eye and the residual stress decreased to zero.

This paper describes the initiation and propagation of heat checking and the variation of residual stress on aluminum die casting dies. Detailed observations by SEM were conducted repeatedly after every prescribed number of shots on the cavity surface near the gate of die casting dies made of steel, SKD61 (AISI H13), in actual service, and the origins of heat checking were searched. Using the newly defined crack density, the propagation of heat checking on the cavity surface was discussed quantitatively. The residual stress on the surface of dies, which was produced by cyclic thermal stress, was measured by X-ray, and the variation of residual stress in the early part of the service life was made clear.The following conclusions were drawn from this investigation.(1) Cracks, appearing in the early stage of heat checking, started from three types of origins: First, pits and wavy patterns, which initiated during the use of the die from the clean surface without any defect before use, second, various machining scratches and defects such as indentations, and third, strain concentrated parts of the die surface.(2) The crack density, obtained from the number of surface cracks longer than 0.2mm, increased rapidly in the first period, and then saturated in the latter period of crack propagation.(3) The residual stress of die surface before use, was compressive stress which had been created during the making of the dies, but it changed immediately toward the tensile side and satulated in the early stage of casting process before 50 shots.

X-ray stress measurement is widely applied to the estimation of residual stress. In real machine parts, residual stress is often observed to have a steep gradient in their near surface layers. In the present study, the effect of the stress gradient on the stress data obtained by the X-ray stress measurement based on the sin2ψ method, was investigated experimentally and analytically.Three kinds of steel specimens were prepared, namely, (A) a shot-peened spring steel plate with decarburized surfaces, (B) a finish ground 0.55%C steel round bar and (C) a shot-peened spring steel plate with non-decarburized surfaces. The residual stress distributions of (A) and (B) specimens showed, respectively, steep negative and positive stress gradients in the near surface layers. But the residual stress near the surface of (C) specimen remained almost constant. The characteristic X-ray used was CrKα radiation.Experimental data for (C) specimen, when plotted on the sin2ψ-cosecθ diagram, can almost be represented by a straight line. But for (A) and (B) specimens, the data on the same diagram nearly fit to the concave and convex curves, respectively.The sin2ψ-cosecθ diagrams of the specimens with a steep stress gradient near the surface were also obtained by calculating the Bragg's angle as the weighted average value of the diffracted X-ray intensity. The analytical results mostly agree with the experimental ones.According to the theoretical analysis of residual stress on the assumption that it varies linearly with depth and σx=σy, the stress obtained by using the sin2ψ method with CrKα radiation was found to correspond to that of the layer 4.4μ deep from the surface.