What heating defects are easy to appear in heat treatment

Heat treatment is an important process to improve the properties of metal materials, and the key of heat treatment clock is undoubtedly the heating process. If errors occur in heating, resulting in heating defects, it will have a serious impact on the metal performance, and sometimes even cause irreparable losses. Then, what are the types of heating defects in heat treatment and what are the reasons for them?

One of the heating defects: overheating
When the temperature of steel material is too high or the holding time at high temperature is too long, the austenite grain coarsening is caused. This phenomenon is called overheating. Austenitic grain coarsening will lead to higher brittleness and lower toughness of steel, increase the tendency of deformation and cracking during quenching, and thus reduce the mechanical properties of parts. The out of control of the furnace temperature instrument is usually the main cause of overheating. Generally, the overheating of steel can refine the austenite grain by annealing, normalizing or multiple high-temperature tempering of the overheated structure.

However, even if the steel material with overheated structure is refined again, there will inevitably be some coarse granular fractures, which is called fracture heredity. This is usually caused by impurities such as manganese sulfide dissolving into the austenite crystal interface when overheating. When steel is impacted, it is easy to fracture along the coarse austenite grain boundary.
When austenitizing heat treatment is carried out again for steel materials with coarse martensite, bainite and widmanstatten structures, even if the steel is heated to the conventional quenching temperature with a slow heating speed, and overheating does not occur, the austenite grains will still show the tendency of coarsening. This phenomenon is called structural heredity. The heredity of coarse structure can be eliminated by intermediate annealing or multiple high-temperature tempering.

Heating defect 2: overburning
In addition to coarsening austenite grains, too high heating temperature will also cause another bad result - local oxidation or melting of grain boundaries. This situation will lead to the weakening of metal grain boundaries, serious deterioration of properties, and cracking during quenching. This phenomenon is called overburning. As overburning involves both physical and chemical processes, once it occurs, the metal structure is difficult to recover, so it can only be discarded. Therefore, in the process of heat treatment, we must avoid overheating due to high heating temperature.

Heating defect 3: decarburization and oxidation
Steel materials with a certain concentration of carbon can enhance the hardness, fatigue strength and wear resistance of metals. However, during heating, the carbon on the steel surface will be oxidized by oxygen, hydrogen, carbon dioxide, water vapor and other substances due to direct contact with the medium or atmosphere, which will reduce the carbon concentration in the steel surface, affect the surface hardness, fatigue strength and wear resistance, and cause residual tensile stress concentration on the steel surface, thus forming surface network cracks. This phenomenon is called decarburization.

Not only the carbon element on the surface of steel will be oxidized, iron and alloy will also be oxidized by oxygen, hydrogen, carbon dioxide, water vapor and other substances in the medium or atmosphere to form an oxide film. This phenomenon is called oxidation. The dimensional accuracy and surface brightness of high-temperature workpiece will be reduced after oxidation, and steel parts with poor hardenability of oxide film are prone to quenching soft spots.
In order to prevent decarburization and oxidation, the surface of steel parts should be packed and sealed with stainless steel foil, heated by salt bath furnace or flame combustion furnace, and the purified inert gas should be used as the protective atmosphere.

Heating defect 4: hydrogen embrittlement
When high-strength steel is heated in a hydrogen rich atmosphere, its plasticity and toughness will decrease. This phenomenon is called hydrogen embrittlement. Hydrogen embrittlement can be avoided by heating in vacuum, low hydrogen atmosphere or inert atmosphere. Hydrogen embrittlement can be eliminated by tempering, aging and other hydrogen removal treatments for workpieces that have appeared hydrogen embrittlement. In some cases, hydrogen embrittlement can also be used to achieve special processing purposes, such as alloy crushing.