Heating defects and control technology

A, overheating phenomenon we know heat treatment in the process of heating the most easily lead to overheating of austenitic grain bulky, decrease the mechanical properties of the parts. 1. General overheating: heating temperature is too high or at high temperature holding time is too long, cause the austenitic grain coarsening called overheating. The coarse austenite grain can reduce steel strong toughness and brittle transition temperature, the increase of quenching deformation and cracking tendency. And the cause of overheating temperature instrument is out of control or mixture ( Often happen to don't understand the process) 。 Overheating by annealing, normalizing or many times after high temperature tempering, in normal circumstances to austenite grain refinement. 2. Genetic: fracture overheating of steel, reheating after quenching, austenitic grain refinement can make, but sometimes still appear rough granular fracture. Over genetic theory of fracture is more, it is generally believed by the heating temperature is too high and make the sundries such as MnS into austenite and enrichment for the interface, and cooling these inclusions can precipitate along crystal interface, easy when rough along austenite grain boundary faults. 3. Bulky organization genetic: there is gross martensite, bainite and westergren steel to austenite of body tissue, with slow heating to conventional quenching temperature, even lower, the austenitic grain size is still a massive, this phenomenon is called genetic organization. Can be used to eliminate the bulky organization of hereditary, intermediate annealing or high temperature tempering treatment many times. Second, burnt heating temperature is exorbitant, not only cause the austenitic grain bulky, and partial oxidation or grain boundary melting, lead to grain boundary weakening, called burnt. Steel performance deteriorated badly after burn, and cracks are formed when quenching. Burnt organization cannot recover, can only be scrapped. So to avoid the happening of burnt in the work. Three steel during heating, decarburization and oxidation, surface carbon and medium ( Or atmosphere) The oxygen, hydrogen, carbon dioxide and water vapor, reduces the surface carbon concentration is called decarburization, decarburization steel after quenching surface hardness and fatigue strength and wear resistance is reduced, and the surface residual tensile stress is the formation of a surface mesh crack. When heated, the surface of iron and steel alloy and element and the medium ( Or atmosphere) The reaction of oxygen, carbon dioxide and water vapor generated oxide film phenomenon known as oxidation. High temperature ( Generally above 570 degrees) Workpiece size accuracy and surface brightness deterioration after oxidation, with oxide film of hardenability steel quenching is seen a soft point of difference. In order to prevent oxidation and the measures to reduce the decarburization are: surface coating, stainless steel heating foil packing seal, use salt bath furnace heating, use protective atmosphere heating ( After purification, the inert gas, control of carbon potential in furnace) , flame combustion furnace, The furnace gas is reducing) Four, hydrogen embrittlement of high strength steel heating in rich hydrogen atmosphere, the phenomenon of the plasticity and toughness decrease called hydrogen embrittlement. Hydrogen embrittlement of workpiece by hydrogen in addition to handling ( Such as tempering, aging, etc. ) Also can eliminate hydrogen embrittlement, with vacuum and low hydrogen atmosphere or inert atmosphere heating can avoid hydrogen embrittlement.