Heat treatment development history
(1) Clean heat treatment
The waste water, waste gas, waste salt, dust, noise and electromagnetic radiation formed by heat treatment will pollute the environment. Solving the environmental pollution of heat treatment and implementing clean heat treatment (or green heat treatment) is one of the development directions of heat treatment technology in developed countries. In order to reduce the emission of SO2, CO, CO2, dust and coal slag, the use of coal as fuel has been basically eliminated, and the use of heavy oil is becoming less and less. Most people switch to light oil. Natural gas is still the most ideal fuel.
The waste heat utilization of the combustion furnace has reached a very high level. The optimization of the burner structure and the strict control of the air-fuel ratio ensure that NOX and CO are reduced to a minimum under the premise of reasonable combustion; gas carburizing and carbonitriding are used And vacuum heat treatment technology replaces salt bath treatment to reduce the pollution of waste salt and CN-containing toxic substances to water sources; uses water-soluble synthetic quenching oil to replace part of quenching oil, and uses biodegradable vegetable oil to replace part of mineral oil to reduce oil pollution.
(2) Precision heat treatment
Precision heat treatment has two meanings: on the one hand, it is based on the use requirements, materials, and structural dimensions of the parts, using physical metallurgy knowledge and advanced computer simulation and testing technology to optimize process parameters to achieve the required performance or maximize the material On the other hand, it fully guarantees the stability of the optimized process, and realizes that the dispersion of product quality is very small (or zero) and the heat treatment distortion becomes zero.
(3) Energy-saving heat treatment
Scientific production and energy management are the most potential factors for the effective use of energy. The establishment of a professional heat treatment plant to ensure full-load production and give full play to equipment capabilities is the choice of scientific management. In terms of heat treatment energy structure, priority is given to primary energy; waste heat and waste heat are fully utilized; processes with low energy consumption and short cycles are used instead of processes with long cycles and high energy consumption.
(4) Less non-oxidizing heat treatment
By using protective atmosphere heating instead of oxidizing atmosphere heating to controllable atmosphere heating with precise control of carbon potential and nitrogen potential, the performance of parts after heat treatment is improved, heat treatment defects such as decarburization, cracks, etc. are greatly reduced, and the finishing allowance after heat treatment is reduced , Improve material utilization and machining efficiency. Vacuum heating gas quenching, vacuum or low-pressure carburizing, nitriding, nitrocarburizing and boronizing can significantly improve quality, reduce distortion and increase life.
Application effect of heat treatment technology
(1) Expanded the scope of application of GCr15 steel
Generally, the effective wall thickness of the ferrule during quenching of GCr15 steel M is less than 12mm, but due to the strong cooling capacity of nitrate salt during BL quenching, if stirring, stringing, and water addition are adopted, the effective wall thickness of the ferrule can be expanded to about 28mm.
(2) Stable hardness and good uniformity
Because BL transformation is a slow process, generally GCr15 steel needs 4h, GCr18Mo steel needs 5h, the ferrule is isothermal for a long time in nitrate salt, and the surface core structure changes almost simultaneously, so the hardness is stable and the uniformity is good. Generally, GCr15 steel BL The hardness after quenching is 59～61HRC, and the uniformity is ≤1HRC. Unlike the larger ferrule wall thickness during quenching, problems such as low hardness, soft spots and poor uniformity will occur.
(3) Reduce quenching and grinding cracks
In the production of railway and rolling mill springs, due to the large size and heavy weight of the ferrule, the M structure is brittle during oil quenching. In order to obtain high hardness after quenching, strong cooling measures are often taken, resulting in quenching micro cracks; while BL quenching, due to The BL structure is much tougher than the M structure, and the surface forms a compressive stress as high as -400～-500MPa, which greatly reduces the tendency of quenching cracks; during grinding, the surface compressive stress offsets part of the grinding stress and makes the overall stress level Drop, greatly reducing the grinding cracks.
(4) Improved spring service life
For railway and rolling mill springs that are subjected to large impact loads, the main failure modes when used after M quenching are: the inner sleeve cracks during assembly, the outer ring ribs fall off during the impact during use, and the inner ring cracks, while the austempered spring Due to the good impact toughness and surface compressive stress, whether the inner sleeve cracks during assembly, or the outer sleeve ribs fall off during use, the tendency of the inner sleeve to crack is greatly reduced, and the edge stress concentration of the roller can be reduced. Therefore, the average life and reliability after austempering are higher than those after M quenching