Heating defects and control

(1) Overheating phenomenon

We know that overheating during the heat treatment process will most easily lead to the coarsening of austenite grains, which will reduce the mechanical properties of the parts.

1. General overheating: The heating temperature is too high or the holding time at high temperature is too long, which causes the austenite grains to coarsen and is called overheating. Coarse austenite grains will reduce the strength and toughness of the steel, increase the brittle transition temperature, and increase the tendency of deformation and cracking during quenching. The cause of overheating is the out-of-control of the furnace temperature meter or mixing (often caused by not knowing the process). The superheated structure can be annealed, normalized or tempered at high temperature for several times, and then re-austenitized under normal conditions to refine the grains.

2. Fracture inheritance: steel with overheated structure, after reheating and quenching, although the austenite grains can be refined, sometimes there are still coarse granular fractures. There is a lot of controversy in the theory of fracture heredity. It is generally believed that impurities such as MnS have been dissolved into austenite and enriched in grain boundaries due to excessive heating temperature, and these inclusions will precipitate along the grain boundaries during cooling. It is easy to fracture along the coarse austenite grain boundary when subjected to impact.

3. Inheritance of coarse structure: When steel parts with coarse martensite, bainite and Widmanite structure are re-austenized, they are heated to the conventional quenching temperature at a slow speed, or even lower, the austenite crystal The grains are still coarse, and this phenomenon is called tissue heredity. To eliminate the heredity of coarse tissues, intermediate annealing or multiple high temperature tempering treatments can be used.

(2) Overburning phenomenon

Excessive heating temperature will not only cause coarse austenite grains, but also local oxidation or melting of the grain boundaries, leading to weakening of the grain boundaries, which is called overburning. The performance of steel deteriorates severely after overburning, and cracks are formed during quenching. The burned tissue cannot be recovered and can only be scrapped. Therefore, avoid over-burning during work.

(Three), decarburization and oxidation

When steel is heated, the surface carbon reacts with oxygen, hydrogen, carbon dioxide and water vapor in the medium (or atmosphere) to reduce the surface carbon concentration, which is called decarburization. The surface hardness, fatigue strength and resistance of decarburized steel after quenching The abrasiveness is reduced, and the residual tensile stress on the surface is easy to form surface mesh cracks.

During heating, the iron and alloy on the surface of the steel react with elements or oxygen, carbon dioxide, water vapor in the medium (or atmosphere) to form an oxide film, which is called oxidation. The dimensional accuracy and surface brightness of the workpieces deteriorate after oxidation at high temperatures (generally above 570 degrees), and the steel parts with poor hardenability of oxide film are prone to quenching soft spots.

Measures to prevent oxidation and reduce decarburization include: coating the workpiece surface, sealing and heating with stainless steel foil packaging, heating with a salt bath furnace, heating with a protective atmosphere (such as purified inert gas, controlling the carbon potential in the furnace), flame burning furnace (Make the furnace gas reductive)

(4) Hydrogen embrittlement

When high-strength steel is heated in a hydrogen-rich atmosphere, the plasticity and toughness decrease is called hydrogen embrittlement. Hydrogen embrittlement can also be eliminated for workpieces with hydrogen embrittlement (such as tempering, aging, etc.). The use of vacuum, low hydrogen atmosphere or inert atmosphere heating can avoid hydrogen embrittlement.