May 10,2023

Heat treatment processes of 21CrMoV5-7 stel specimens and the piston head made of the same steel were investigated. Theresults show that the transformafon products in the quenched 21CrMoV5-7 steel after austenizing were two kinds of martensite with diferentmorphologies , including lath martensite and acicular martensite, in the range of testing teniperalure , the strength , impact absorbed energand ductility of the 21CrMoV5-7 stel raises first with the increase of quenching temperature , reaches the peak values near 910℃ ,and thenalls down. When austenized at 910℃  and oil quenched then tempered at 650℃  ,the 21CrMoV5-7 steel piston head can meet therequirements of mechanical properties.

The piston in the engine is mainly composed of two main parts, the piston head and the piston skirt, which are transmitted to the connecting rod through the piston pin to drive the crankshaft to rotate. The piston head is also an integral part of the combustion chamber. Under the conditions of high temperature, high pressure, high speed, and poor lubrication, it is subjected to impact, torsion, and strong pressure. Therefore, the influence of different heat treatment states of piston materials on processing distortion was verified and verified, and the following conclusions were formed:

1) The internal stress of different types and states of TC4 alloy raw materials has a decisive influence on the processing distortion of large thin-walled flange parts, and its influence is far greater than the influence of cutting residual stress;

2) The initial residual stress of TC4 alloy raw materials cannot be eliminated by ordinary annealing and stress relief annealing heat treatment. Double annealing heat treatment can well eliminate the initial residual stress of its material; The cast iron raw materials are interspersed with stress-relieving annealing in the process of adding T to eliminate the processing stress; when the annealed raw materials in the form of iron plates and wrought iron rings are selected, they must be processed after double annealing

The piston in the engine is mainly composed of two main parts, the piston head and the piston skirt, which are transmitted to the connecting rod through the piston pin to drive the crankshaft to rotate. The piston head is also an integral part of the combustion chamber. It works under the conditions of high temperature, high pressure, high speed, and poor lubrication to withstand impact, torsion, and strong pressure. Therefore, the influence of different heat treatment states of piston materials on processing distortion was verified and verified, and the following conclusions were formed:

1) The internal stress of different types and states of TC4 alloy raw materials has a decisive influence on the processing distortion of large thin-walled flange parts, and its influence is far greater than the influence of cutting residual stress;

2) The initial residual stress of TC4 Taiwan gold raw materials cannot be eliminated by ordinary annealing and stress relief annealing heat treatment, and the double annealing heat treatment can well eliminate the initial residual stress of the material;

3) For TC4 alloy large and thin-walled parts, it is preferred to use cast iron raw materials after double annealing for T-adding process, interspersed with stress-relief annealing to eliminate processing stress; when choosing annealed raw materials in the form of iron plates and wrought iron rings, double annealing treatment is required followed by subsequent processing.

As far as the material is concerned, it is required to maintain sufficient strength and elasticity at the working temperature, and it has good scratch resistance, wear resistance, toughness, and fatigue resistance. Our company used 42CrMo4 steel to produce piston head hair damage, but found that the 42CrMo4 steel piston head of a certain marine diesel engine is prone to local ablation and thermal cracking due to high operating temperature during operation. The piston head of the diesel engine needs to use materials with better heat resistance to reduce similar failure phenomena. The 21CrMoV5-7 steel is a quenched and tempered, heat-resistant structural steel, the German material grade is 1.7709. The steel has good hardenability and can obtain good strength, plasticity and toughness after quenching and tempering treatment 2). Although the nuts and pads in the D19 diesel engine are made of 21CrMoV5-7 steel 3] the heat treatment process basically refers to the heat treatment system in Table 8 of DIN17240-1976: 890 ~ 940 ° C heating oil quenching 680 ~ 720 ° C tempering and air cooling. However, the performance of 21CrMoV5-7 steel piston head is very different from heat-resistant and high-heat-resistant nuts and bolts. In order to understand the heat treatment process of 21CrMoV5-7 steel, the heat treatment process of 21CrMoV5-7 steel was studied, and the heat treatment of 21CrMoV5-7 steel piston head was trial-produced. The test materials and methods used the annealed 21CrMoV57 steel as the sample material. The sample ruler intercepted part of the material from the center of the sample. Should be f120 mm x 200 mm. The chemical composition analysis results of the material were analyzed by direct reading spectrometer 06 (COLUMBUS) in Table 1.

Machined from raw material (dimensions 120 mm x200 mm)

ø16 mm x200 mm sample to be heat treated. After the samples are heat treated according to the following heat treatment parameters, the round bar tensile samples with a gauge length of 8 mm, Charpy V-type impact samples (10 mm x 10 mm x 55 mm) metallographic samples and hardness samples are processed.

In order to study the effect of quenching temperature on the microstructure and mechanical properties of 21CrMoV5-7 steel, the box furnace was selected to heat at 820, 860, 890, 910, 940°C and 960°C for 30 minutes and then oil quenched; after quenching, it was kept at 700°C for 60 minutes. Fire treatment, air cooling to room temperature after tempering. In order to study the influence of tempering temperature on the microstructure and mechanical properties of 21CrMoV5-7 steel, the box furnace was selected to heat at 910°C for 30 minutes, and then oil quenching was selected for tempering at 500, 550, 600, 630, 660°C and 690°C for 60 minutes. Air cool to room temperature after tempering.

The tensile strength, yield strength, elongation and reduction of area were measured by electro-hydraulic universal testing machine SHT4305 (30606008); the impact absorbed energy was tested by universal testing machine; the microstructure was observed by EVO18 scanning electron microscope and XJL09BD metallographic microscope ; Determination of Brinell hardness with Brinell hardness machine HB3000B.

Test results and analysis 22.1 Effect of ignition temperature on the microstructure and properties of 21CrMoV57 steel The sea-fire hardness of 21CrMoV57 steel after oil quenching at 820, 860, 890, 910, 940°C and 960°C is 375, 390, 405, 410, 413, 415HB respectively The quenching structure is shown in Figure 1:

The quenching hardness increases with the increase of the quenching temperature. The higher the heating temperature, the more fully the carbide dissolves into the austenite. The higher the carbon content in the austenite, the higher the hardness of the martensite obtained by quenching. For the composition range of steel: ≤0.6C, ≤4.9Mn, ≤5Cr, ≤5Ni, ≤5.4Mo (mass fraction%) can be used according to the empirical formula Ac; (C) =910 -203C12 -15.2Ni+44.7Si + 104V + 31.5Mo + 13.1W; Ac, (C) =723 -10.7Mn -13.9Ni +29Si + 16.9Cr +290As +6.38W Rough calculation 4] Ac, =740.8CAc879.5 C of 21CrMoV57 steel. After quenching at 820 C, the black block in the microstructure in Fig. 1 (a) is undissolved ferrite, most of which are ferrite and pearlite, and there is little martensite, indicating that the austenitization at 820C is not sufficient. 8600 After deep annealing, the microstructure in Fig. 1(b) is martensite, ferrite and pearlite, which are partially austenitized when heated; lath martensite can be seen in the microstructure after quenching at 890, 910, 940, and 960°C and acicular martensite; the carbon content (mass fraction) in austenite is 0.20% ~ 0.40%, mainly lath martensite (accounting for about 80%) s] there is also a small amount of acicular martensite, gray matrix It is lath martensite and the jujube-like structure is acicular martensite; the martensite lath structure can be clearly seen in the sample quenched at 960 C under a higher magnification of the scanning electron microscope (Fig. 1 ( )); Compared with the typical lath martensite, the long axis direction of the jujube martensite is shortened, and the width is slightly expanded; during the formation of the lath martensite, carbon redistribution occurs, resulting in carbon-rich austenite microdomains, Transforms into high carbon acicular martensite during cooling7. Although the microstructure in Fig. 1(c) is martensite, the microstructure is uneven and there are many massive and large acicular martensite; the microstructure in Fig. 1(d) is uniform and fine martensite; As the quenching temperature increases, the martensite lath becomes coarser, especially in the microstructure of Fig. 1(f), the martensite structure is very coarse, and the martensite lath is very long. The austenite grain size after fire at 820, 860, 890, 910, 940, and 960°C is grade 5, grade 6, grade 8 respectively according to GB/T 63942002 "Method for Determination of Average Grain Size of Metals".