Study on the shaping and thermal remediation of extremely toughened bolts

The technical requirements for the design of cylinder head bolts and connecting rod bolts are the highest. The data analysis is as follows: 1) Rapid cooling with water after tempering can reduce the temper brittleness of steel and increase the impact value. 2) If the tempering temperature is too low, even if the tempering is repeated after multiple tempering, the KU5 cannot be more than 40J3.) The full potential of the 35CrNi3WA steel is still unable to meet the technical requirements.

Since 35CrNi3WA can not meet the design requirements, we have selected 30NCD16 steel for series process test 30NCD16 chemical composition heat treatment process test method and results: 1) heat treatment routine process test (salt furnace quenching + well furnace tempering) results.

For the 30 conventional process tests of the cylinder head bolts, 860880 oil quenching was used respectively, and after tempering, it was quickly cooled, and several normalizing pretreatments were also made for comparison. Although some oil quenching process processors meet the mechanical properties required by the design, the tensile strength is unqualified after the tempering temperature is increased, and the tempering temperature impact value is unacceptable, so the processability is poor. The unconventional process quenching temperature for the cylinder head bolt is 840, and the quenching medium is water and CaCl2 saturated aqueous solution. It can be seen from the comparison test results that the 840 quenched CaCl2 saturated aqueous solution and 550 water-cooled twice can meet the mechanical properties required by the design. Moreover, the tempering temperature is about 10 degrees above and below, and the mechanical properties can also be qualified and the process is strong.

Analysis of the oil quenching situation, to improve the quenching temperature (the same tempering temperature), no significant effect on the strength; increase the number of tempering water cooling, the impact value is not obvious; increase the tempering temperature, the impact value increases, but exceeds Critical temperature, the strength is not qualified. It is confirmed by the search data that the impact value is not high due to the temper brittleness, and the increase of the quenching temperature will increase the temper brittleness. Obviously, the increase in temper brittleness is caused by grain growth. Different steels have different temper brittleness tendencies, which are mainly determined by the chemical composition of the steel. The inclusion of only impurities such as carbon and phosphorus is not sufficient to cause temper brittleness, but in steels such as chromium-nickel, an increase in phosphorus content will strongly increase temper brittleness. Adding molybdenum or tungsten to it can weaken the temper brittleness tendency, and adding 0.4% 0.6% molybdenum or 1.5% tungsten can achieve the best effect. The addition of molybdenum or tungsten does not really eliminate the temper brittleness, but only changes the kinetics of the temper brittle process, which greatly slows down its development. Reducing the quenching temperature, prolonging the tempering holding time, increasing the number of tempering and adopting the method of quick cooling after tempering all contribute to further reducing the temper brittleness of steel <1>.

Conclusion For high-strength and toughness bolts on medium-speed marine diesel engines, such as cylinder head bolts and connecting rod bolts, the technical requirements are very high and the correct material selection is essential. Through the series of tests of 35CrNi3W and 30NCD16, 30NCD16 was finally selected.

Although the W or Mo element in the 35CrNi3W and 30NCD16 steels reduces the temper brittleness of the steel, it does not completely eliminate the temper brittleness. Reducing the quenching temperature, prolonging the tempering holding time, increasing the number of tempering and using the method of quick cooling after tempering all contribute to further reducing the temper brittleness of the steel. Practice has proved that the heat treatment process of 840 quenched CaCl2 saturated aqueous solution + 550 water cooling twice can meet the technical requirements of the upper cylinder head bolt and connecting rod bolt of medium speed diesel engine, and the processability and reproducibility are strong.

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