RU2113509C1 - Method for heat treatment of connecting rod forgings from alloyed steels - Google Patents

Abstract

FIELD: metallurgy. SUBSTANCE: method includes heating of forgings up to authentication temperature, hardening, high-temperature tempering and descaling. Heating is carried out up to temperature of 860-960 C in oxidizing atmosphere with excess air coefficient of 1.25-2.25 for 60-180 min. Hardening is carried out by cooling with cooled compressed air for 6-20 min down to temperature below 320 C. Heating of connecting rod forgings from steel 40XH2MA is carried out to temperature of 900±10 C for 144±5 min with excess-air coefficient of 1.4±0.1. Cooling is lasted for 10-12 min to temperature below 320 C, and tempering is conducted at temperature of 560±10 C for 240 min. EFFECT: higher quality of articles and reduced expenditures for heat treatment. 2 cl, 1 tbl

Description

 The invention relates to metallurgy, in particular to the heat treatment of steel forgings from alloy steels, mainly connecting rods.

 A known method of hardening of steel products, including heating to an austenitizing temperature and cooling at a speed satisfying the condition.

;
где
a, b, c - коэффициенты зависимости порогового значения вязкости разрушения от скорости охлаждения образцов;
K_Irth = a/(lgV+b)+C;
K_Irth - пороговое значение вязкости разрушения стали МПа•м^1/2;
V - скорость охлаждения изделия, ^oC/с;
L - определяющий размер дефекта, мм;
σ $\genfrac{}{}{0ex}{}{\text{макс}}{\text{р}}$ - максимальное растягивающее напряжение, МПа;
m - коэффициент местонахождения дефекта;
Способ трудно реализуем на практике, требует проведения значительного числа экспериментов в лабораторных условиях для различных плавок. Главное, что он не устраняет дефектов, а предлагает регламентировать скорость охлаждения для предотвращения их распространения [1].

;
Where
a, b, c are the coefficients of the dependence of the threshold value of fracture toughness on the cooling rate of the samples;
K _Irth = a / (logV + b) + C;
K _Irth - threshold value of fracture toughness of steel MPa • m ^1/2 ;
V is the cooling rate of the product, ^o C / s;
L is the defining size of the defect, mm;
σ $\genfrac{}{}{0ex}{}{\text{Max}}{\text{R}}$ - maximum tensile stress, MPa;
m is the defect location coefficient;
The method is difficult to implement in practice, requires a significant number of experiments in the laboratory for various swimming trunks. The main thing is that it does not eliminate defects, but suggests regulating the cooling rate to prevent their spread [1].

A known method of heat treatment of connecting rods of automobile engines, which consists in heating to an austenitization temperature of 830 ^o C, holding for 0.5 h, cooling in antifreeze, heating to a temperature of 650 ^o C, holding for 1 h, slow cooling to normal temperature and cleaning the scale steel shot [2].

 The disadvantage of the above method is that in the presence of surface defects in the forging during hardening, cracking occurs and an unrecoverable defect is obtained. In addition, expensive coolant antifreeze is consumed. In the process of using antifreeze, it becomes contaminated with scale and burns out organic components, which leads to the stratification of the coolant into antifreeze and water. Thus, hot forgings made of alloy steel, in contact with water in separate areas, experience high thermal stresses, resulting in the formation of cracks even in the absence of stress concentrators on the surface. Cases of general marriage up to 20% are observed. Immediately change the antifreeze to a new one.

 The task is to develop a method for the heat treatment of connecting rods of alloy steels, which will eliminate surface defects (stress concentrators) during heating and aging the forgings for austenization and apply hardening with a low cooling rate to reduce thermal stresses. In addition, it is necessary to get away from the use of expensive coolant with properties that change during operation.

The technical result in the method of heat treatment of forged alloy steel, comprising heating to the austenitizing temperature, quenching, high temperature tempering, descaling is achieved in that the heating is carried out until the temperature of 860 ^o C960 ^o C, in an oxidizing atmosphere with a coefficient of excess air of 1.25 ^o C 2.25 for 60 ^o C180 min, and hardening is carried out by cooling with compressed air for 6 ^o C20 min to a temperature below 320 ^o C.

Heating forgings of steel rod 40HN2MA lead to a temperature of 900 ± 10 ^o C for 144 ± 5 min with excess air ratio 1.4 ± 0.1, cooling is carried out within 10 minutes ^o C12 to a temperature below 320 ^o C, and lead rental at a temperature of 560 ± 10 ^o C for 240 minutes

Heating of forgings in an oxidizing atmosphere with an excess air coefficient of more than 1.25 (12.5 m ^{3 of} air per 1 m ^{3 of} gas) leads to the burning out of defects on the forgings surface, thereby eliminating stress concentrators that open during hardening under the influence of thermal stresses and lead to the final marriage. It was experimentally established that heating with a lower coefficient of excess air does not completely eliminate surface defects. When heated with an excess air coefficient of more than 2.25, heavy scale formation occurs and during austenization not only the surface defect-containing layer burns out, but also the base metal of the forging. In addition, the efficiency of the furnace is reduced (it is necessary to heat a lot of excess air).

An increase in the temperature of quenching heating to 860 ^o C 960 ^o C allows increasing the degree of alloying of austenite, which leads to a decrease in the upper critical rate of quenching, and this, in turn, allows to obtain the structure of quenching martensite during cooling at low speed, using cooling with compressed air in within 6 ^o C20 minutes In addition, when heated to quenching to 860 ^o C960 ^o C, the oxidation rate of steel exceeds the rate of decarburization of the surface layer.

 The heating time for austenitization depends on the size of the forging. Small forgings are enough to heat for 60 minutes, and large forgings need to be heated for 180 minutes. With shorter heating times, the austenitization process is incomplete.

 The cooling time of the forgings with compressed air also depends on their size. Small forgings are sufficient to cool for 6 minutes, and large forgings need to be cooled for 20 minutes. With shorter cooling times, the martensitic transformation is incomplete.

 A comparative analysis of the proposed technical solution with the prototype shows that the proposed method of heat treatment of forgings is different from the prototype.

If the prototype method leads to heating up to 830 ^o C, then in the proposed method to 860 ^o C960 ^o C. In the prototype method, the atmosphere is not regulated, and in the proposed method creates an oxidizing atmosphere with a coefficient of excess air supply of 1.25 ^o C 2, 25. In the prototype, the exposure time when heated for 30 minutes, and in the proposed method 60 ^o C 180 minutes In the prototype, hardening is carried out by immersing the forgings in antifreeze, and in the proposed method, by cooling with compressed air for 6 ^o C20 minutes

 These distinguishing features ensure the elimination of defects at the stage of heating the forgings and reduce thermal stresses during the hardening process, which leads to the production of high-quality products.

 Thus, this technical solution meets the criterion of "novelty."

 The analysis of copyright certificates, patents and scientific and technical information did not reveal the use of new significant features of the invention according to their functional purpose. Thus, the present invention meets the criterion of "inventive step".

An example of a specific implementation. Processing according to the proposed method was subjected to forgings of the connecting rod of a KAMAZ automobile drawing 740-1004050 / 55, made of a bar with a diameter of 70 mm, steel grade 40XH2MA TU 14-1-2598-79. The mass of forgings is 4.3 kg. The size of pilot lots of 720 pieces each. Forgings were laid in one layer of 15 pieces per pallet (lots 1 and 2), in two layers on a pallet (lot 3) and heated in an oxidizing atmosphere with an air supply excess coefficient of 1.4 ± 0.1 for 144 min to 900 ± 10 ^o C. Then it was cooled with compressed air for 10 min in an air-cooling chamber to a temperature below 320 ^o C. Next, the forgings on pallets were transferred to a tempering furnace, where tempering was carried out for 5 minutes at a temperature of 560 ^o C. After that, the forgings were overturned in containers and cooled in calm air. The cooled forgings were cleaned with steel shot in shot blasting plants and subjected to visual inspection, hardness control and luminescent inspection for cracks and hairline. To clean the quality of the connecting rods, the microstructure and mechanical properties were determined.

 The test results are shown in the table.

 For comparison, data on the prototype and the requirements of GOST 8479-70, KP 640 are given.

 The microstructure was evaluated metallographically. The microstructure of the connecting rods of the batch 1, 2 and prototype - finely dispersed sorbitol, batch 3 - medium-sized sorbitol. The grain size of the connecting rods of parties 1, 2 and 3 N 8 GOST 5639-82.

 Analysis of the experimental data showed that the heat treatment of the proposed method can eliminate cracks and get quality products.

Claims (2)

1. The method of heat treatment of forgings of a connecting rod made of alloy steels, including heating to austenitic temperature, hardening, high-temperature tempering, descaling, characterized in that the heating is carried out to a temperature of 860 - 960 ^o C, in an oxidizing atmosphere with an excess air supply coefficient of 1, 25 - 2.25 for 60 to 180 minutes, and hardening is carried out by cooling with compressed air for 6 to 20 minutes to a temperature below 320 ^o C. 2. The method according to claim 1, characterized in that the heating of the forgings of the connecting rod from steel 40XH2MA is carried out to a temperature of 900 ± 10 ^° C for 144 ± 5 minutes with an excess air coefficient of 1.4 ± 0.1, cooling is carried out for 10 - 12 minutes to a temperature below 320 ^o C, and leave at 560 ± 10 ^o C for 240 minutes

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