42CrMo is a medium carbon alloy structural steel with good comprehensive mechanical properties, good hardenability and wide application.
It is often used in the machinery industry to manufacture gears, connecting rods, high-strength bolts and other important parts.
42CrMo steel forging stock is supplied by a steel plant.
The manufacturing process is as follows: refining outside the furnace → ingot casting → forging stock → normalizing.
A company purchased this batch of forged blanks for manufacturing the coiling shaft in the strip production rewinding unit. The final heat treatment process is quenching and tempering.
The mechanical property requirements are tensile strength 900 ~ 1100MPa, yield strength ≥ 650MPa, and impact absorption energy ≥ 40J;
During the warehousing inspection of raw materials, it was found that the macro and micro metallographic structures were abnormal, and it was preliminarily judged that the forging blank was not normalized.
In this post, the methods and possibilities of improving the quality of the forging stock are discussed and analyzed according to the physical and chemical test results by formulating several heat treatment schemes.
2. Physical and chemical test
1. Chemical composition analysis
The test material is 42CrMo steel forging stock, which is accepted according to the requirements of high-quality steel in alloy structural steel (GB / t3077-2015).
The test results are shown in Table 1 by Spectrolab spectrometer and meet the standard requirements.
Table 1 chemical composition (mass fraction) of forging stock sample
|Element||Standard value||Measured value|
2. Metallographic examination
Carry out metallographic inspection for the raw materials in the warehouse.
After sampling according to the requirements of GB / T 13298-2015 metallic microstructure inspection method, assess the macrostructure defects and non-metallic inclusions according to GB / T 1979-2001 structural steel macrostructure defect rating diagram and GB / T 10561-2005 determination of non-metallic inclusion content in steel.
The macro morphology is shown in Fig. 1 and the microstructure is shown in Fig. 2. See Table 2 for test results.
Fig. 1 low magnification inspection of forging blank sample (1:1 industrial hydrochloric acid hot corrosion)
a) Coarse reticular ferrite
b) Part of the tissue is sorbite
c) Part of the tissue is composed of pearlite + a small amount of bainite and ferrite
d) Part of the tissue is coarse
Fig. 2 metallographic structure of forging blank sample
Table 2 metallographic test results of forging billet sampling
|Test items:||Detection result|
|Low magnification defect||Generally, the porosity is grade 1, and the dendrite on the test surface is coarse and has serious segregation|
|Organization type||The tissue distribution is extremely uneven, the tissue is coarse, and the tissue is ferrite + pearlite + reticular ferrite + a small amount of bainite|
3. Mechanical property test
The forging stock shall be sampled and tested for mechanical properties according to the requirements of GB / T 2975-2018 sampling location and sample preparation for mechanical properties test of steel and steel products.
The results are shown in Table 3.
Table 3 test results of mechanical properties of forging stock
|Tensile strength / MPa||Yield strength / MPa||Body length rate after fracture (%)||Impact absorption energy (KV2 / J)|
|983||845||12.0||10, 6.0, 6.0|
4. Comparison between heat treatment scheme and physical and chemical test results
According to the physical and chemical test results, the forging billet has coarse structure, abnormal structure and poor mechanical properties, which should be due to poor forging quality and no proper normalizing treatment.
In order to study the influence of raw material structure on final heat treatment (quenching and tempering), verify the improvement degree of various heat treatment processes on raw material quality, formulate heat treatment scheme, analyze and propose improvement measures.
Now, three heat treatment processes – normalizing, quenching and tempering, normalizing + quenching and tempering are developed to test the forging stock residues.
The samples are taken from the forgings after heat treatment to test their metallographic structure and mechanical properties.
The test results are compared with the design requirements. The results are shown in Table 4.
Table 4 performance indexes and structure types after three processes
|Process status||Tensile strength / MPa||Yield strength / MPa||Elongation after fracture (%)||Impact absorption energy (KV2 / J)||Organization type|
|Normalizing||1098||959||12.5||17, 15, 18||Segregation exists in the organization; The structure is sorbite + troostite + bainite|
|Tempering||878||752||16.0||77, 87, 80||There is obvious segregation in the tissue; The structure is sorbite + a small amount of ferrite|
|Normalizing + tempering||988||828||16.0||94, 107, 110||Segregation exists in the organization; The tissue is sorbite + a small amount ferrite|
|Design requirement||900~1100||≥650||≥12.0||≥40||Sorbite, a small amount ferrite allowed|
Through analysis and comparison, the strength and toughness of the normalized specimen are significantly improved, and the network ferrite in the structure disappears, as shown in Fig. 3;
Fig. 3 structure after normalizing (880 ℃ air cooling)
Although the net ferrite is eliminated in the specimen directly quenched and tempered by forging stock, the tensile strength is far lower than the technical requirements and the structure segregation is obvious, as shown in Fig. 4;
Fig. 4 Tempering (860 ℃ oil cooling and 610 ℃ air cooling)
The normalizing + quenching and tempering samples not only meet the technical requirements in all performance indicators, but also improve the uniformity of the structure, as shown in Fig. 5.
Fig. 5 normalizing + quenching and tempering (880 ° C air cooling + 860 ° C oil cooling, 610 ° C air cooling)
The abnormal structure produced in the forging stock is usually caused by the high temperature during forging, the rapid growth of the grain, and the failure to refine the grain during forging.
At the same time, the insufficient cooling rate after forging leads to the production of network ferrite, which will seriously deteriorate the mechanical properties of the material, especially the impact toughness at room temperature.
In order to prevent the generation of coarse grains, network ferrite and even widmanstatten structure, the heating temperature during forging should be strictly controlled, and the cooling rate must be reasonably controlled.
After normalizing, the microstructure can be further refined to prevent the residual of network ferrite and other structures.
The forging stock has not been normalized as required, and the metallographic inspection shall be further strengthened during the warehousing of raw materials to ensure that it meets the organization requirements under normal process.
If it is found that there are abnormal phenomena such as coarse structure, reticular ferrite or even widmanstatten structure in the original structure, normalizing treatment must be carried out before quenching.
Otherwise, the structure and properties obtained will not meet the expected requirements directly, and even the existence of reticular ferrite may cause quenching cracks during quenching, resulting in product failure during heat treatment.