Knowledge

Deformation

1.1 Causes

1) Asymmetric shape or significant thickness difference of the workpiece

2) Large mechanical processing stress that was not eliminated before quenching

3) Uneven heating and cooling

4) Improper heating and clamping method of the workpiece

5) Transformation of the quenched microstructure

1.2 Preventive and remedial measures

1) Improve the structural design of the workpiece, select materials rationally, adjust the machining allowance, and add process holes

2) Add preheating or stress relief annealing processes

3) Use multiple operation methods such as multiple preheating, precooling quenching, double liquid quenching, step quenching, and isothermal quenching

4) Support and bind the workpiece for quenching heating properly

5) Straighten the deformed workpiece

 

Low hardness

2.1 Causes

1) Mixed raw materials

2) Low heating temperature and short holding time

3) Slow cooling rate

4) Excessively high heating temperature and long holding time, which increases the stability of austenite and leaves a large amount of retained austenite after quenching

5) Decarburization on the surface of the workpiece during heating

6) Excessive impurities in the steel

2.2 Preventive and remedial measures

1) Perform spark testing on the steel

2) Follow the normal quenching process specifications. Before re-quenching, perform normalizing or annealing treatment first

3) Cool at a rate greater than the critical cooling rate

4) Use cryogenic treatment to increase hardness

5) Regularly deoxidize and remove slag from the salt bath, or use protective atmosphere heating

6) Select steel that meets technical requirements

 

 

Cracking

3.1 Cracks before quenching: Oxidation and decarburization can be observed on both sides of the cracks after quenching, and the fracture surface is black.

3.1.1 Causes

Improper rolling or forging, resulting in shrinkage cavities, layers, and white spots, etc.

3.1.2 Preventive and remedial measures

Strictly control product quality to ensure the qualification of raw materials.

 

3.2 Cracks caused by cooling: The fracture surface is red, oily or purple.

3.2.1 Causes

1) Non-metallic inclusions in raw materials are segregated in a band or network pattern.

2) Raw materials are mixed.

3) Inhomogeneous cooling, stress concentration, complex workpiece shape, uneven thickness, sharp corners, bends, and machining marks.

4) Improper cooling and inappropriate selection of cooling medium.

5) No annealing treatment between repeated quenching and no timely tempering.

 

3.2.2 Preventive and remedial measures

1) Use forging to reduce the level of carbides, and adopt pre-cooling, graded quenching, isothermal quenching, etc. during heat treatment, and temper in a timely manner.

2) Perform spark testing.

3) Improve the design to ensure uniform thickness and no defects that cause quenching cracks.

4) Select reasonable cooling media and quenching methods.

5) Anneal before repeated quenching.

 

3.3 Cracks caused by brittleness

3.3.1 Causes

1) Excessive quenching temperature, causing overheating of the structure, bright white fracture surface, and coarse grains.

2) Severe segregation or non-spheroidized carbides in the original structure.

3.3.2 Preventive and remedial measures

1) Strictly control heating temperature and follow the process documents, and strengthen the inspection of the microstructure during the process.

2) Perform normalizing treatment or spheroidizing annealing.

 

 

Soft spots

4.1 Causes

1) Local decarburization on the surface of the workpiece or contamination with dirt

2) Impurities in the quenching medium or excessive use temperature

3) Poor cooling capacity of the cooling medium

4) Inappropriate cooling method for the workpiece, with mutual contact between workpieces

5) Improper pre-heat treatment, leaving a large amount of large ferrite blocks in the steel

4.2 Preventive and remedial measures

1) Select an appropriate pre-heat treatment process

2) Keep the medium clean, cool down reasonably, and prevent decarburization of the workpiece

3) Replace the quenching cooling medium

4) Cool the workpieces separately

5) Re-quench, but normalizing or annealing treatment should be carried out before re-quenching

 

 

Decarburization

5.1 Causes

1) Heating in an oxidizing atmosphere

2) Poor slag removal during salt bath deoxidation

3) Excessive heating temperature and prolonged holding time

5.2 Preventive and remedial measures

1) Use protective atmosphere heating or surface coating protection

2) Regular slag removal during salt bath deoxidation

3) Follow the process specifications

4) For quenched parts that have undergone decarburization, apply carburizing methods

 

 

Corrosion

6.1 Causes

The sulfate content in the salt bath exceeds the range specified by the process.

6.2 Preventive and remedial measures

1) Select heating salt that meets technical requirements.

2) Use magnesium-aluminum alloy or charcoal to remove sulfate from the salt bath.

 

 

Vigor team has more than 20 years experience in casting, forging, cold forming processes and the post treatment, as well as a robust surface treatment supply chain. If anything we can help or any parts you want to develop, please contact us at info@castings-forging.om