A method of soft annealing high carbon steel

Abstract: A method for soft annealing of high carbon steel, characterized by taking objects to be soft annealed directly from a hot forming step and cooling to below A1-20°C, heating the objects to A1+20°C or above, and then cooling the objects down to beneath the A1 temperature of the steel quickly as in air, which step is performed at least once, heating the objects to A1+20°C or above, cooling the objects down to about 740°C, and then cooling the objects down to about 690°C at a cooling rate of 3.5 °C/min. or lower, and finally cooling the objects down to ambient temperature.

Description: Background Soft annealing normally takes 12 - 48 hours time and is performed batchwise or continuously in an oven. The load in the oven is then heated to about 800°C which takes between 2 and 10 hours, the temperature is maintained for about 2 hours, the temperature is then quickly brought down to about 790°C and then down to about 690°C at a rate of about 10°C/hour. This procedure is very time consuming, costly and may result in decarburisation. Further, because of different conditions at different locations in the oven, the structure will vary substantially between the objects, and also within one and the same object. A test of a batch of tubes of standard steel SAE52100, showed that the hardness varied between 170 and 220 HB, depending on where in the oven the respective tube was placed. When soft annealing a batch of tubes one tube can be subjected to different conditions over the length, resulting in thermal stresses, and in a considerable distortion at the subsequent hardening. There is a great need of reducing the costs involved in the soft annealing process for high carbon steel. However, the structure of steel after the soft annealing process is of critical importance for the subsequent procedures, and for the intended use. Many attempts have been made to develop the soft annealing procedure in different aspects. According to JP04103715-A (Sumitomo Metals Ind.) high carbon chromium bearing steel is subjected to spheroidising treatment; first by heating to 780-820°C and cooling to below Ar1b point at less than 200°C/hr and by heating to Ac1b-(Ac1b+40)°C, cooling to below Ar1b at less than 200°C/hr, heating to Ac1b-(Ac1b+40)°C and cooling to below Ar1b at less than 75°C/hr. This publication deals mainly with the structure of the steel, and does not teach how to solve the problems discussed above. The Invention The object of the invention is to provide a method for soft annealing of objects made of a high carbon steel with which the above mentioned drawbacks are eliminated. More specifically, one object of the invention is to shorten the process time and to make possible an in-line operation, while obtaining a very small decarburization. Another object is to provide a method for soft annealing high carbon steel which gives little or no perlite at the surface, and which results in fewer and smaller carbides at the surface, and a smaller structural gradient. Yet another object is to provide a method of soft annealing which can be performed in-line, and wherein the objects are exposed to identical conditions, and thereby an unitary structure and unitary characteristics are obtained. This is achieved with the method according to the invention, which is characterized by taking objects to be soft annealed directly from a hot forming step and cooling to below A1-20°C, heating the objects to A1+20°C or above, and then cooling the objects down to beneath the A1 temperature of the steel quickly as in air, which step is performed at least once, heating the objects to about A1+20°C or above and quenching down to about 740°C, and then cooling the objects down to about 690°C at a cooling rate of 3.5 °C/min. or lower, and thereafter cooling the objects down to ambient temperature. In total this process takes about 1.5 hours. The objects are taken directly from the hot forming step, and are transferred separately in-line into a soft annealing oven. The oven can be divided into a number of chambers, with intermediate spaces, in which the air cooling takes place, possible enhanced by sprinkling water onto the objects. The method is fast, continuous and can be performed in-line. The conventional transport and logistic problems are eliminated. One heating cycle from ambient temperature to 650°C disappears, as well as heating at 820°C for 2 hours. Less decarburization takes place and a smaller structural variations results. With the method according to the invention, only a small part of the carbides is dissolved each time, resulting in that there is less carbon in solution which shall diffuse. Very important advantages are obtained using the method according to the invention. A substantial amount of energy is saved by using the hot forming heat in the subsequent soft annealing step. Further, the in-line system reduces the required oven capacity several times, and it is less labour intensive. According to one embodiment of the invention the oven configuration consists of a number of parts corresponding to the number of heating and cooling cycles of the process, arranged one after the other with intermediate empty spaces, in which air cooling, possibly forced air cooling using water sprinkling, takes place and wherein the tubes are transported with their longitudinal direction perpendicularly to the longitudinal direction of the oven, i.e. the direction of movement, and preferably using carriers rolling the tubes through the oven. This eliminates the need of a separate straightening step for the tubes after the soft-annealing. Brief description of the Drawings The invention will now be described more in detail with reference to the appended drawings, in which fig. 1 shows a graph temperature vs. time illustrating a possible soft annealing method according to the invention. Definitions A1 is upon heating defined as the temperature when the matrix phase transforms into austenite. A1 is upon cooling defined as the temperature when the austenite phase will transform into other products. Example A possible soft annealing heat treatment cycle is shown in Figure 1. The hot rolled SAE52100 steel component was heated in a furnace as quickly as possible to a temperature above A1 + 20°C in this case 820°C. When it reached this temperature it was brought out in air and cooled to a temperature below A1 - 20°C in this case 650°C. The component was again heated to a temperature above A1 + 20°C (810°C) and brought out in air to cool. Finally, the component was heated to a temperature above A1 + 20°C (800°C). After this it was transported into a temperature zone in the furnace with lower temperature for controlled cooling. The temperature was lowered relatively quickly to 740°C using fans in the furnace. After this the cooling from 740 to 690°C was made in 20 min. Table 1 shows the structure classified according to the German standard SEP 1520 and hardness. Most material user accepts these values. SEP 1520 and hardness. CG PA CN Brinell Hardness 2,1 3,0 4,0 199

Claims: A method for soft annealing of high carbon steel, characterized by taking objects to be soft annealed directly from a hot forming step and cooling to below A1-20°C, heating the objects to A1+20°C or above, and then cooling the objects down to beneath the A1 temperature of the steel quickly as in air, which step is performed at least once, heating the objects to A1+20°C or above, cooling the objects down to about 740°C, and then cooling the objects down to about 690°C at a cooling rate of 3.5 °C/min. or lower, and finally cooling the objects down to ambient temperature. A method according to claim 1, characterized by heating the objects to A1+20°C or above and then cooling the objects down to below the A1 temperature of the steel in air at least twice.

Inventors: LARSSON STAFFAN, LUND THORE, OELUND PATRIK, LARSSON, STAFFAN, LUND, THORE, OELUND, PATRIK, Ölund, Patrik
Applicants: OVAKO STEEL AB
Application year: 1999
Main classification: C21D1
IPC codes: C21D1, C21D8, C21D6