METALLURGICAL CHANGES IN STEELS DUE TO CRYOGENIC PROCESSING & ITS APPLICATIONS
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METALLURGICAL CHANGES IN STEELS DUE TO CRYOGENIC PROCESSING & ITS APPLICATIONS Metallurgical changes in steels due to cryogenic processing & its applications Abstract: Cryogenic processing is a supplementary process to conventional heat treatment process in steels. It is an inexpensive one time permanent treatment affecting the entire section of the component unlike coatings. Though the benefits have been reported widely, there are issues debated upon, in respect of the treatment parameters, extent of benefits experienced in different materials, underlying mechanism and pretreatment conditions. A study on the improvement in wear resistance and the significance of treatment parameters in different materials has been made. It is found that cryogenic treatment imparts nearly 110% improvement in tool life. It is even superior to tin coatings. The underlying mechanism is essentially an isothermal process. Keywords: Cryo processing; Wear resistance INTRODUCTION: The word Cryogenics is derived from the Greek words 'Kryos" (meaning cold) and "Genes" (meaning born). The cryogenic processing is modification of a material or component using cryogenic temperatures. Cryogenic temperatures are defined by the Cryogenic Society of America as being temperatures below 120K (-244F, -153C). Cryogenic processing makes changes to the crystal structure of materials. The major results of these changes are to enhance the abrasion resistance and fatigue resistance of the materials. The thermal treatment of metals must certainly be regarded as one of the most important developments of the industrial age. One of the modern processes being used to treat metals (as well as other materials) is cryogenic tempering. Until recently, cryogenic tempering was viewed as having little value, due to the often brittle nature of the finished product. It is only since the development of computer modeled cooling and reheats curves that the true benefits of cryogenically treated materials have become available to industry and the general public. Cryo tempering is a permanent, non-destructive, non-damaging process (not a coating) which reduces abrasive wear (edge dulling), relieves internal stress, minimizes the susceptibility to micro cracking due to shock forces, lengthens part life, and increases performance.
The small & hard carbide particles within the martensitic matrix help support the matrix and resist penetration by foreign particles in abrasion wear. The reported large improvements in tool life usually are attributed to this dispersion of carbides in conjunction with retained austenite transformation. . This cryogenic processing step causes irreversible changes in the microstructure of the materials, which significantly improve the performance of the materials. The treatment calls for a precise temperature control during the processing, usually up to one-tenth of one degree, necessitating elaborate controls and sophisticated instrumentation. Further explanation to the "Concrete effect" is as follows: Cryogenic treatment of alloy steels causes transformation of retained austenite to martensite. Freshly formed martensite changes its lattice parameters and the c/a ratio approaches that of the original martensite. Etta (h) carbide precipitates in the matrix of freshly formed martensite during the tempering process. This h carbide formation favors a more stable, harder, wear-resistant and tougher material. This strengthens the material without appreciably changing the hardness (macro hardness). The other major reason for the improvement is stress relief. The densification process leads to an elimination of vacancies in the lattice structure by forcing the material to come to equilibrium at -196'C and lowering the entropy in the material. This lower entropy leads to the establishment of long range order in the material which leads to the minimization of galvanic couples in the material thus improving the corrosion resistance of materials including Stainless Steels. Besides, there is some amount of grain size refinement and grain boundary realignment occurring in the material. These two aspects lead to a tremendous improvement in the electrical and thermal conductivity of the material thus transporting the heat generated during the operation of the tool away from the source and increasing its life. Because austenite and martensite have different size crystal structures, there will be stresses built in to the crystal structure where the two co-exist.
> Cryogenic Processing is relatively new Conclusion: Cryogenic Processing is not a substitute for heat-treating. Cryogenic Processing is not a coating. It affects the entire volume of the material. It works synergistically with coatings. These benefits extend to cast iron, aluminum, stainless steels, and other materials. The scope of cryogenics has expanded widely from basic military and space applications to various civil applications. Cryogenic processing is mainly applicable to steels. Cryogenic treatments can produce not only transformation of retained austenite to martensite, but also can produce metallurgical changes within the martensite. this offers many benefits where ductility and wear resistance are desirable in hardened steels While various experts dispute the benefits of time-at-temperature control; available research, along with a correlation with standard heat treating processes indicates that this control is the key to maximizing the potential of cryogenic tempering. As is the case with many scientific discoveries, the cost factor limits the usefulness of this process in the production phase of the materials industry. Recent advancements: Currently extensive research is being conducted in an effort to better the available cryocooler technology in fields like materials for the regenerator, cylinder heads, etc., refrigerants used, size of cryocooler, increasing the efficiency. Stirling technology is used to produce miniature cooling systems Superconductivity occurs in a wide variety of materials, including simple elements like tin and aluminium, various metallic alloys, some heavily-doped semiconductors, and certain ceramic compounds containing planes of copper and oxygen atoms is achieved by this cryogenic processing . dilution refrigerators is One important application of superfluidity and this state is also achieved by crogenic processing Cryogenics has been successfully tested on flexible circuits to reduce the residual stress between layers of the circuit. This helps keep the circuit from curling and separating Treated transformers show a lack of hysterisis. The magnetic core saturates less. Ball and roller bearings respond beautifully to cryogenic processing. Increases of wear life of two to three hundred percent are not uncommon. Cryogenically treated resistance welding electrodes will last about 3 to six times longer than untreated electrodes.
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