IDENTIFICATION OF CRACK APPEARANCE BY SOUND EMISSION DURING STEEL QUENCHING
Heat treatment, especially quenching and tempering often represent almost the final stage of manufacturing process in the production of machine components. Only the most exacting piece of work like grinding, polishing etc. follows. Final mechanical properties, such as residual stresses and hardness layer, therefore depend on optimum parameters of a quenching process and monitoring of the process itself. Even then happens that some work-pieces failed also due to cracking of the material. Detecting such faults in the due time may reduce unnecessary costs due to final machining of the machine part or even due to damage of the machine occurred due to machine part breakdown. The paper treats an experimental setup comprising detection of sound emission in the course of quenching process. Due to heat transfer from a specimen’s surface to a quenching medium, film boiling and nucleate boiling occur round a heated specimen, which strongly affects quenching. An investigation of sound emission in the quenching process was carried out with cylindrical specimens made of chrome-molybdenum heat-treatable steel 42CrMo4 quenched in different quenching media. Sound-pressure signals demonstrated by different amplitudes depending on time at different frequencies are shown in 3D diagrams. It has turned out that an analysis of sound-emission signals can provide useful information that confirms differences occurring in quenching with different quenching media, different specimen’s shapes and under different quenching conditions. Analysis of sound emission signals and selection certain signals which differ from the regular sound emission can provide useful information regarding cracking with certain step of probability and confirm the differences occurring in quenching with different quenching conditions. The results lead to the applicability of the new approach to the control of the hardening processes of steels.
Overview of distortion and residual stress due to quench processing part I: factors affecting quench distortion
The most frequently cited heat treating problem is distortion control, particularly distortion control problems related to the non-uniform thermal gradients that typically accompany quench processing. The roles of various factors affecting quenching distortion and steel cracking including phase transformation behaviour of steel, retained austenite, quench media and process selection, quench severity, and the importance of optimising quench uniformity are discussed. Finally, quench process simulation methodology and the potentially enormous benefits that they provide in system and process design methodology are discussed, as is the use of computational fluid dynamics (CFD) analysis in quench system design.
Quenching properties of parts having stress concentrators
In heat treatment of steels, and especially in quenching, it is important to prevent crack formation. Macro- and microcracks can form in quenching both on the surface and inside the parts. Cracks appear most often in parts from tool steels with stress concentrators. Methods of immersing parts with stress concentrators in a quenching medium that reduce the probability of crack formation are described in the present work.