The quenching and annealing process of the spindle as well as common defects and prevention methods

The spindle will always encounter some problems of this kind during the heat treatment process. For example, some common defects are prone to occur during annealing and quenching and tempering. When encountering these defects, how should we solve them? What is the process of shaft heat treatment? Zhengzhou Gou's gave a brief introduction to these issues.

① Defects easily caused by complete annealing and how to prevent them

a. The hardness is too high. Because the cooling rate after annealing is too fast, the generated flaky pearlite is too thin, resulting in an increase in hardness. It is not conducive to cutting. The solution is to reheat and reduce the cooling speed (the cooling speed should be ≤120°C/h).

b. The reason for the appearance of coarse massive ferrite in the structure is that the cooling rate is too slow, and the cooling rate should be controlled above 30°C/h.

② Defects easily caused by quenching and tempering treatment and how to prevent them

1. Serious decarburization on the surface of the workpiece

This is caused by the fact that the workpiece is heated for too long and the temperature is too high in a heating furnace with an oxidizing atmosphere. The solution is to reasonably select the heating temperature and holding time, or choose heating equipment such as protective atmosphere heating furnaces and vacuum furnaces.

2.Quenching crack

The reasons are as follows: 1. Internal defects in raw materials; 2. Too intense quenching and cooling; 3. Stress concentration at sharp corner grooves and thick cutting lines; 4. Decarburization of the workpiece surface.

The corresponding solutions are: 1. Strengthen the inspection of raw materials entering the factory; 2. Change from single-medium cooling to water-oil or water-air double-liquid quenching cooling method; 3. Improve the design or cutting process; 4. Improve the heat treatment heating process.

3. Mechanical properties are too low

This is because the quenching heating temperature is low and the free ferrite is not completely dissolved into the austenite; or the raw material steel has poor hardenability; or the tempering temperature is too high or too low. The corresponding solutions are: adjust the quenching temperature; select alloy structural steel with higher carbon and alloy element content according to the hardenability curve; adjust the tempering temperature.

Heat treatment process:

1.Quenching and tempering treatment

The purpose is to obtain high strength and high toughness combined with good comprehensive mechanical properties. It is suitable for: a. Medium carbon structural steel workpieces or semi-finished products that require higher comprehensive mechanical properties; b. Softening of certain high-alloy steels; c. Improving the surface roughness of semi-finished products and reducing the deformation tendency of final heat treatment and quenching, etc. . In this process:

a. Quenching heating and heat preservation should ensure that the original structure is completely austenitized.

b. The quenching cooling rate should ensure that most of the austenite transforms into martensite, and the ferrite volume fraction on the surface of the part should not exceed 3% to 5%.

c. The high temperature tempering temperature of quenching depends on the hardness requirements of the steel type and sequence for the quenching and tempering treatment and the hardness after quenching.

d. The holding time of high-temperature tempering depends on the furnace capacity and steel type. Generally, it is appropriate to hold for 30 to 40 minutes after thorough firing.

2. Complete annealing

Its purpose is to eliminate structural defects and internal stresses such as coarse grains or uneven grain sizes that often exist in forgings (castings, weldings, hot-rolled parts, etc.), so that the strength, plasticity and toughness of the steel can meet the technical requirements, that is, uniform structure and fine grain size. It can transform grains, eliminate internal stress, improve cutting performance, etc., and prepare the structure for final heat treatment. Suitable for softening treatment of hypoeutectoid and eutectoid non-alloy steels and alloy steels (castings, forgings and weldments). What we should do when performing this process operation:

a. In addition to strictly controlling the heating temperature and time, surface oxidation and decarburization of steel parts should be minimized. The depth of the oxidative decarburization layer shall not be greater than 1/2 of the machining allowance of the blank.

b. After complete annealing, heating and through-burning, when a large number of furnaces are installed, the furnace is slowly cooled or a certain cooling rate and discharge temperature are limited. When single pieces or small batches are produced, they can be heated and thoroughly burned before being discharged and buried in white ash or Slowly cooling in the ashes of vegetation.

3. Stress relief annealing. Its purpose is to eliminate the residual internal stress generated during castings, forgings, weldments and cutting processes. It can reduce the hardness of raw materials or blanks to facilitate cutting processing. It can also completely eliminate the residual internal stress of the processed parts.