Heat treatment process of connecting rod

1.Manufacturing process route of connecting rod

The general manufacturing process of quenched and tempered steel connecting rod is: forging → quenching and tempering → shot blasting → hardness and surface inspection → correction → fine pressing → flaw detection → machining → finished product.

Many factories use forging waste heat quenching + high temperature tempering to replace quenching and tempering. After tempering, hot correction replaces cold correction to reduce correction stress.

The non quenched and tempered steel connecting rod shall be controlled cooled according to the controlled cooling curve after forging to obtain stable mechanical properties that can replace quenching and tempering.

2.Quenching and tempering process of connecting rod

See Table 1 for the quenching and tempering process of common carbon steel and alloy structural steel connecting rods.

UseSteel gradeHeating temperature ℃Cooling modeHeating temperature ℃Holding time minCooling modeHardness HB
Cars, jeeps and small tractors40Cr860+10Oil cooling620±1060Water cooling211-298
Cars, jeeps and small tractors15840±10Add oil after 12s of brine670±10Salt bath 30Air cooling207-211
Cars, jeeps and small tractors45Mn2810-860Oil cooling620-640162Water spray228-269
Cars, jeeps and small tractors40Final forging 95060-100 ℃ hot oil 35S610±10120Air cooling217-280
Cars, jeeps and small tractors45810-830Air cooling after 5S of brine580-600120Air cooling228-269
Trucks and tractors40MnB850±10Oil cooling650±10120Air cooling220-260
Trucks and tractors55Final forging 900Oil cooling650±10180Water cooling229-269
Heavy duty vehicle40Cr850±10Oil cooling610±10210Water cooling223-280
Heavy duty vehicle40CrMoA860±10Oil cooling550±10180Air cooling223-280
Heavy duty vehicle40SiMn860±10Oil cooling560±10150Air cooling223-280
High horsepower diesel engine18CrMo870±10Oil cooling610-630240Air cooling298-321
High horsepower diesel engine12CrMo860-870Oil cooling550-580180Air cooling311-331

3.Waste heat quenching and tempering process for connecting rod forging

Using forging waste heat quenching can not only simplify the process and save energy, but also improve the cutting performance and mechanical properties. See Table 2 for the comparison of mechanical properties of connecting rods made of 40Cr and 45 steel after quenching and tempering, forging waste heat quenching and tempering.

Steel gradeHeat treatment processσb MPaφ (%)δ (%)αK (longitudinal / transverse) (J · cm2)Hardness (HB)
40Cr1160 ℃ forging residual heat quenching → 660 ℃ tempering85666.119.5166/94252-260
40Cr850 ℃ quenching → 610 ℃ tempering79965.620.6163/59249-255
451180 ℃ forging residual heat quenching → 600 ℃ tempering91458.219.3123216
45850 ℃ quenching → 550 ℃ tempering87737.418.2121235

ω (C) Medium carbon steel forgings of 0.40-0.55% are usually quenched by forging waste heat. The following points shall be noted during mass production:

1) The forging heating temperature should be 1100-1220 ℃, for example, the forging heating temperature of 45 steel can be selected in the range of 1150-1220 ℃.

2) In actual production, the final forging temperature is the quenching temperature, which is generally 900-1050 ℃. During operation, the residence time from final forging to oil quenching shall be controlled to prevent ferrite precipitation.

3) The quenching oil temperature and the stay time of connecting rod in oil shall be controlled.

4) In order to prevent cracks caused by long storage time after quenching, tempering shall be carried out in time after quenching.

For mass production, induction heating is an economical and effective blank heating method. Because the accurate control of temperature can ensure the stability of final forging temperature, it is especially suitable for forging waste heat quenching process. This should be given priority in plant design (new plant design or old plant technical transformation).

Application example: the connecting rod of 488 engine is made of 40mn2s, with chemical composition (mass fraction):0.38%c, 1.50%mn, 0.074%s, 0.018%p, 0.33%si and 0.2%cr. The rolling temperature of steel is 1180-1210 ℃, and the final rolling temperature is 850 ℃. The specification of rolled stock is Φ 40mm and Φ Two types of 35mm are used for trial production of connecting rod body and connecting rod cap respectively. Forging production process: blanking → induction heating → roll forging blank → hydraulic die forging → trimming → residual heat quenching → tempering → strong shot blasting → flaw detection → fine pressing → hardness inspection → weight inspection. The roll forging temperature is 1250-1280 ℃, the quenching temperature is 900-950 ℃, and the tempering temperature is 600-610 ℃. The microstructure of the forgings is tempered sorbite plus a small amount of free ferrite, and the microstructure grade is grade 1. The depth of decarburization layer on the surface of forgings after heat treatment is 0.05-0.08mm. The streamline of metal macrostructure is consistent with the shape. The hardness of connecting rod is 229-255hbs, and the hardness difference is 26hbs. The overall tensile breaking load of connecting rod forgings is 131.0kn. After shot peening, the surface residual compressive stress of connecting rod forgings is between 240-300mpa. The connecting rod forgings have good cutting performance and qualified weight tolerance. The tensile and compressive fatigue strength limit of the trial produced connecting rod is 286.2mpa, and the safety factor of the connecting rod body is 1.87-2.23. No abnormality was found in engine mapping test and road test.

The connecting rod of a diesel engine is made of 40MnB, the billet specification is 55 square steel, and the forging heating adopts medium frequency induction heating. The heating temperature is 1200-1250 ℃, the quenching temperature is about 900 ℃, and the tempering temperature is 620-630 ℃. The quenching medium is quenching oil, the oil temperature is 40-80 ℃, and the residence time in the oil shall not be less than 10min. Hardness: 444-578hbs after quenching and 255-302hbs after tempering. Mechanical properties: σ s=816MPa, σ b=932MPa, δ= 18%, φ= 57.5%。

4.Controlled cooling process of non quenched and tempered steel connecting rod

At the initial stage of application, the non quenched and tempered steel shows slightly poor toughness and unstable cutting performance. This is not only related to the steel and forging process, but also the cooling control after forging, which can not guarantee the stability to obtain appropriate microstructure and mechanical properties.

According to the requirements of large-scale production of automobile industry for the performance stability of connecting rod, Beijing Institute of mechanical and electrical research has developed a controlled cooling production line of non quenched and tempered steel connecting rod, which can be used for the controlled cooling of automobile connecting rod to realize the automatic control of cooling speed in the range of 20-120 ℃ /min according to the design curve.

The structure of non quenched and tempered steel is ferrite plus pearlite, and carbide and other precipitates are distributed on the ferrite matrix. The cooling rate in the temperature range of 750-550 ℃ determines the relative amount of ferrite and pearlite. Fast cooling speed can inhibit the precipitation of ferrite, the amount of pearlite is large and fine, and it can also inhibit the coarsening of precipitates, resulting in high strength and low plasticity. Slow cooling enables ferrite to be fully analyzed. Meanwhile, pearlite will be relatively coarsened, which improves the toughness and plasticity and reduces the strength. Therefore, better comprehensive properties of strength and toughness can be obtained by controlling the cooling rate. For example, the test shows that the forging heating temperature of 35MnVS steel should be 1050-1220 ℃, and the cooling rate after forging should be 60-100 ℃ /min.

If 43mns is used to produce car connecting rod, the forging heating temperature is 1150-1250 ℃, and the final forging temperature is 900-1100 ℃. The control cooling of connecting rod after forging shall be carried out in the pass through roller chain furnace, and the furnace temperature shall not be lower than 800 ℃. The equipment is divided into three zones for temperature control. Zones 1 and 2 have heating and cooling functions. Zone 3 is only cooled, and the temperature is controlled at 250-550 ℃. The connecting rod runs in the furnace for 12-15min. The temperature of the small end of the connecting rod shall be lower than 400 ℃ when it is discharged from the furnace, and air cooling after it is discharged. The production rate of cooling equipment is controlled to be 480-720 pieces / h. After the above “controlled forging and cooling” treatment, the hardness of connecting rod forgings is 217-255hbs, the structure is flake pearlite plus network ferrite (bainite and martensite are not allowed), and the grain size is: grade 6-7 to grade 4 from the surface to the center of small end and large end; The rod body is of grade 6-7.