Reasons and countermeasures for crack defects in heavy cast iron bed guide rails using medium frequency induction heating machines for quenching heat treatment

Heavy machine tools have a large cast iron bed and a large cross-sectional area of the bed guide rails. In production, often due to improper induction heating process or poor sensor design, as well as casting structural defects and other reasons, the workpiece will crack or even crack when using an intermediate frequency heating machine for induction quenching, seriously causing the bed to be scrapped. Improvement measures are as follows:

1) For the quenching of large-section cast iron bed guide rails, double-circuit inductors are often used and magnets are installed. The dual-circuit inductor has front-stage preheating, rear-stage quenching heating and water spray cooling. The sensor width is 32mm. Calculated at a quenching operating speed of 6mm/s, the time from preheating to water spray cooling at any heated point is approximately 5s, which ensures that the bed is heated evenly. During operation, attention should be paid to the moments of starting and stopping. If the operation is incorrect, a lateral low temperature zone with a width of about 10mm will easily occur in the gap area of about 12mm between the two poles. During quenching, several longitudinal cracks with a length of 10-20mm will occur in this area. This is because the surface stress of cast iron itself is large, the structure contains a lot of primary cementite, and the volume fraction of Fe3C is >1%. Single-circuit sensors can be used during production to prevent the occurrence of lateral low-temperature zones, but attention should be paid to controlling the temperature at the moment of starting the car during operation. If the heating temperature is low, the above-mentioned cracks will still appear; if the temperature is too high, local overburning of the guide rail may easily occur and cracks may occur.

2) Improper overlapping of two (more) quenchings of a large-section lathe can easily cause quenching cracks. For example, the C61125A bed guide rail surface has a width of 300mm and a length of 6m. The maximum width is 210mm in one quenching using a 100kw medium frequency heating furnace. The guide rail surface with a width of 300mm needs to be quenched in two times. After the first quenching, there was an overlap at 10mm. Due to the superposition of two thermal stresses at the overlap, it was not easy to relax and release in a short time, resulting in serious longitudinal cracks. If a lengthened inductor is used, if the length of the second heating and quenching inductor is 165mm, the 10mm non-hardened area in the first quenching can be eliminated to make it hardened without cracks, but a 10mm annealing zone will appear in the first quenching area. Later, oblique cracks 20-50mm long will occur in this area. Countermeasures: A 150mm long inductor can be used for quenching and heating so that there is no heat-affected zone in the middle during the two quenchings to avoid stress superposition and thus no quenching cracks will occur.

In another case, if the guide rail is very long and requires transverse joints, there is a 15-20mm transverse annealing zone in the overlap area, which is prone to several short longitudinal cracks. Countermeasures: The non-hardened area overlap 5mm close to the quenching area can be used to avoid the peak tensile stress area and reduce the tendency of cracks and cracking in the overlap area.

3) In order to prevent quenching cracks and distortion, the quenching layer should not be too deep, and it is better to make the surface layer dominated by phase change stress. If 8kHz medium frequency induction quenching is used, a quenching layer depth of 2mm is more suitable. At this time, the distortion is very small and cracks are rarely found; if it exceeds 3mm, the cracks will increase a lot, the distortion will increase significantly, and the stress will also become mainly thermal stress. .

When medium-frequency induction quenching is used for large-section cast iron bed guide rails, the quenching operating speed is generally controlled at 6.0-6.1mm/s, and the depth of the hardened layer is 1.8-2.0mm, which can obtain higher quenching quality.

4) Casting defects are the source of medium-frequency induction quenching cracks in the bed guide rails. There are flocculent, massive, dotted and fine flake graphite in the casting structure, which is characterized by multi-angular and obvious, and is a typical micro-stress concentration area, crack initiation area and diffusion area. Serious problems often occur during medium-frequency induction quenching. Longitudinal cracks. Countermeasures: Carry out stress relief treatment twice after rough machining and semi-finishing of the bed (guide rail); at the same time, use high-power rapid quenching during medium-frequency induction quenching to make the depth of the hardened layer <2mm.

This article briefly introduces the causes and measures for cracks in heavy cast iron bed guide rails when they are quenched with a medium frequency induction heating machine. I hope it will be helpful to your heat treatment work. If you want to know more detailed information, you can read more books on heat treatment. I believe you will gain a lot.