Abstract
This paper mainly introduces the causes and countermeasures of the cracking of high chromium cast iron caused by thermal treatment. High chromium cast iron is a common material used in many areas, so the study of its thermal-treated cracks to provide a theoretical basis for practical work.
In this paper, the heat treatment of high chromium cast iron is analyzed and the geneces of cracking are discussed in detail. We also analyzed several methods used to reduce cracking in heat treatment, and gave a summary of the key. The content of this paper provides a reliable knowledge base for the reduction and prevention of crushing cracking in high chromium cast iron.
Introduction
High chromium cast iron is a kind of cast iron with high alloying element chromium and high hardness. High chromium cast iron is widely used in mines, abrasive industry, chemical and other industrial fields. The material has high abrasion resistance and load-bearing capacity. It is because of these advantages that high chromium cast iron is selected as the main material. But in the process of heat treatment of high chromium cast iron, it will often form an internal network-shaped a/s of carbon, which is easily cracked when it is cooled after being heated to a high temperature. It is an important factor affecting its mechanical properties, so studying the causes and countermeasures of the cracking of high chromium cast iron caused by thermal treatment is important to improve its performance.
(1)Cause analysis of cracking
1.Thermal shock
The thermal shock is mainly caused by the sudden cooling or sudden temperature change during the heat treatment of the castings. The alloy has a high contraction rate after heating and cooling, and the sudden cooling or temperature change is difficult to make the internal material of the castings reach equilibrium. The resulting internal stress will deform or even crack the castings.
2.Grain size
The grain size of castings is related to the set temperature of furnace and the heat preservation time of furnace, and has a great influence on the micro-defects and crack of castings. When high chromium alloy is heated to a certain temperature, grain growth will be caused, the casting grain size will become larger, and its internal stress will be increased, then cracks will be caused when being cooled down.
(2)Countermeasures to reduce cracking
1. Control the temperature rise and fall
The temperature rise and fall are divided into three stages, namely raising and falling stages, lasting stage and cooling stage, in which the rate of temperature rise and fall is slow in first two stages, and switched quickly in the cooling stage. The transition of temperature must be done in suitable time to reduce the thermal shock of alloy, during which it should adopt the technology of distributed heating and slow or stepwise cooling.
2. Optimize the cooling rate
The cooling rate is one of the important factors affecting the micro crack in the process of heat treatment. It is necessary to optimize the cooling rate in different temperature zones in order to reduce the thermal shock of the alloy and reach the right cooling speed.
3.Adopt a slow stepwise cooling
The cooling rate of castings is usually very fast in the cooling process, so it is recommended to use a slow stepwise cooling to reduce the thermal shock of low alloy castings, and reach a balance between thermal shock and grain size, in order to avoid the occurrence of mechanical defects.
Conclusion
In this paper, the causes and countermeasures of the cracking of high chromium cast iron caused by heat treatment are introduced. Thermal shock, grain size and cooling rate are analyzed. Several methods of reducing cracking in heat treatment have been proposed, such as controlling the temperature rise and fall, optimizing the cooling rate, and adopting slow stepwise cooling, which provide reliable theoretical basis for the practical work.
