Shang Jimin, Wang Honggang
(Dongbei Special Steel Group – Beiman Special Steel Co., Ltd, Qiqihaer Heilongjiang 161041 China)

Abstract: This paper describes the studies of bearing steel that heating temperature directly affects the amount, the distribution and the size of residual carbon in austenite, and the uniformity of the final spheroidizing structure, under the condition of continuous cooling, and the relationship was discussed between heating temperature and decomposition temperature and transformation structure. Heating temperature can play a leading role on the degree of spheroidizing and uniformity of spheroidizing structure.

Keywords: spheroidizing structure; carbide; austenitic; sorbate; eutectoid decomposition.

There are several ways to use the spheroidized structure with heat treatment obtain carbide. Which uses an isothermal transformation of the annealing method. In the metallurgical enterprises in the high-carbon chromium bearing steel ball annealing the most widely used. The spheroidizing annealing of high carbon chromium bearing steel is the most widely used in metallurgical company. Ball annealing of bearing steel is for good cutting performance and hardening. It is one of the most important processes for fatigue performance, that after tempering. And there are strict uniform spherical structures. Therefore, it was observed in the spheroidization organization whether there were flaky pearlite or sorobody tissue and residual carbide mesh. The size of spherical carbide particles and whether the degree of uniformity meets the standard requirements.

To get uniform spherification. Combine annealing production process. In this paper, the annealing process of continuous cooling is studied and analyzed. The effect of heating temperature and primitive tissue on the behavior of carbide. The relationship between the behavior of the carbides and the relationship between the heating temperature and the decomposition temperature, the relationship between the heating temperature and the transformation of the structure during the continuous cooling annealing is briefly described.

1 test method

The bearing steel of hot rolling condition is used for inspection. Specifications for the Փ 11, Փ 15, Փ 21 bar sample, with the annealing furnace normal annealing and in the corresponding specifications and fixed bar. Remove some of the samples in the box-type electric furnace for high and low temperature annealing. This is carried out by the actual production and simulation of the annealing furnace and box-type electric furnace combined test analysis. During the annealing process, the samples were taken for water quenching before cooling, and the change of the microstructure was observed. In the experiment, metallographic microscope observation and transmission electron microscopy were used to observe and analyze.

2 test technology

According to the production process of table 1, t The organization after annealing is very uneven. In order to find out the corresponding annealing organization. The heating temperature in the test process is 860 ° C(1580F) and 760 ° C(1400F)

Chart 1 Segment temperature℃(F)

cooling method	Roll speed / mm	1	2	3	4	5	6	7	8	9	10	11	12
Continuous cooling	2.52	820	820	810	780	760	740	730	720	710	700	690	650
continuous	2.5	810	810	810	780	760	740	730	720	710	700	690	650

Chart 2 Test process

First group	Heating temperature / ℃	860	840	820	810	780	760	740	730	720	710	700	690	650
	Holding time / minutes	90	60	60	60	90	60	60	60	60	60	60	60	60
Second Group	Heating temperature / ℃	760	750	740	730	720	710	700	690	650
	Holding time / minutes	240	90	90	60	60	90	90	60	60

3 Test results and discussion

Choosing the right heating temperature plays a leading role in strengthening the uniformity of the organization. No matter which spherification annealing process, the high or low temperature of the heating temperature directly affects number of residual carbide, distribution, andgranular size and homogeneity of the final spheroidized tissue. See it in the experiment when the heating temperature at 860 ℃, the austenite decomposition temperature is decomposed at 740 to 710. In the decomposition before the high temperature of 860 ℃ has exceeded the theoretical heating temperature of bearing steel 840 ℃. At this time the carbide dissolves too much, the residual carbon particles remain, in some locality parts of the formation of carbide particles very little uniform and non-uniform carbon concentration of the district, while the entire austenite matrix concentration increased. When continuous cooling, with the temperature to reduce the carbon in the austenite with residual carbide for a sufficient time to agglomerate grow. This process starts from 860 ° C to 760 ° C before the decomposition of 120 ° C slowly cooling the temperature range, so that the residual carbide has sufficient time for agglomeration growth. So to 760 ℃ when the carbide was significantly thicker, resulting in the carbide particles coarse, uneven organization of the phenomenon, which is one of the characteristics of high heating temperature.

When the temperature dropped to 740 ℃. Electronysis analysis began in some small areas to occur eutectoid analysis under high power transmission electron microscopy. The first thing you see a place where large particles of carbide gather and along the austenite boundary are fused, Carbide mesh particles is first decomposed. Because of the accumulation of residual carbide. The carbon concentration of the austenite around these large particles is lower than elsewhere. The temperature of eutectoid decomposition is improved. Therefore, in the presence of large particles of carbide first began to break down, so that part of the spheroidized carbide particles coarse, resulting in uneven organization.

That part of the decomposition in the back. Due to the high temperature of 860℃ heating, a small area of uniform and non-uniform carbon concentration is formed. These compounds have high carbon content and the austenite is more stable. The concentration of unevenly carbon concentration near by 720℃ eutectoid decomposition is too high. The concentration fluctuating too much. And then we’re going to decompose the front region, it forms a black mass of soris. In the A uniform carbon concentration, the lamellar pearlite is formed after the decomposition of 710℃. Therefore, in the continuous cooling process, the spheroidization of large particles of carbide and laminated pearlite forms. The beginning of the decomposition and reduce the decomposition of the end of the temperature, which also shortened the aggregation after the aggregation

The heat preservation time of the ball evenly. The group has not been adequately brought up and improved. There was no significant difference between the group that had decomposed 710℃ and the final group of 650 after a continuous cooling. The resulting spherification tissue is very uneven. Large carbide particles + black-like sorbite, such as flaky pearlite, which is exactly the same as the spheroidized structure in the annealing furnace, indicating that this is a typical superheat organization.

Observed to see the heating temperature of 760 ℃ and 860 ℃ when the organization is obvious different. From 760 ℃ down to 750 ℃ began to break down to 730 ℃ when the decomposition end, higher than 860 ℃ decomposition and the end. This is because the heating temperature is low, austenite dissolved in less carbide, carbon content is low and uneven. Fine carbide spots a lot, in the continuous cooling conditions without heating temperature 860 ℃ stability. At the higher 750 temperature, the decomposition is started. But also because the decomposition before the analysis of time is short. Carbide network  fusing after residual carbide has no time to fully grow up. In the meantime, the homogenization and non-uniform carbon concentration of the cell does not exist. A large number of small carbide particles in the original tissue are transformed when eutectoid decomposition. The activation function in phase change process is lower. Eutectoid decomposition are easy to carry out. The decomposition temperature was increased by 10 and 20 respectively. Make the initial and final eutectoid decomposition temperature rise by 10 and 20 respectively.

In the organization after eutectoid decomposition. Black group of sorbite is made up of small spheres of carbon. 860 ° C formed by the black mass of sorbite is the carbon concentration as the core of the dense particles formed by the composition. So the heating temperature is difference between 860℃ and 760℃. There are obvious differences in spheroidization and morphology. In the former organization, large carbide particles and lamellar pearlite seriously affected the uniformity of spheroidized tissue. The latter due to the low heating temperature, the ball in the organization without significant large carbide particles and flake pearlite, the spheroidized tissue slightly more. Both of the black mass of sorbite, in the process of spheroidization in the state of glue, and it is difficult to achieve uniform spheroidal organization.

4 conclusion

(1). The continuous cooling and spheroidizing annealing process at a heating temperature of 860 ° C resulted in a structure of a large carbide particle + sheet pearlite + dense carbide particles consisting of a black mass of sorbate. Spheroidization is very uneven and is a typical superheated tissue.

The heating temperature is 760 continuous cooling ball annealing process. The resulting tissue is a globular pearlite + fine spherical carbide composition of the black group of sorbite, the heating temperature is low but no significant lack of heat of the original sheet pearlite.

Heating temperature increases, reduce eutectic decomposition temperature; heating temperature decreases, improve eutectoid decomposition temperature.

References:
[1] 卢先熙《金属学教程》；
[2] 刘云旭《金属学与热处理原理》。