Bainite hardening

Abstract: A method of complete bainite hardening of steel for use in bearings and other load carrying components, wherein bainite transformation is performed at a temperature just above the martensite formation temperature, transforming 25 - 99 % of the austen-ite into bainite at said temperature, and then increasing the temperature to speed up the transformation of the remaining austenite into bainite.

Description: The present invention relates to a method for complete bainite hardening of steel for use in bearings and other load carry-ing components. Background of the invention Bainite hardening is sometimes preferred instead of martensite hardening. This is because a bainitic structure in general has better mechanical properties, e.g. higher toughness, higher crack propagation resistance etc.. Therefore, for critical components such as bearings or other load carrying components especially subjected to fatigue a bainitic structure could be most suitable. Bainite hardening is widely used to produce components with high strength and hardness. For a given steel the bainite structure often exhibit superior mechanical properties and structural stability compared to martensite. The disadvantage with bainite hardening is the long process time. To reduce the processing time for isothermal bainite transformation it is necessary to increase the bainite transformation temperature. However, this will reduce the hardness which could affect the properties of the component. GB,A, 2019436 discloses a method of producing a steel alloy which is tough, strong, and economical. The method involves a bainite hardening treatment in the region of 0 and 100°C above the martensite point, until a maximum transformation of 85%, preferably 80%, and then cooled to room temperature. The method is more economical as there is a great saving in time compared to a traditional complete transformation. However, the part of the steel which is not transformed into bainite does affect the properties of the steel. US-A-5650027 discloses a method for producing a high - carbon steel wire rod and wire excellent in drawability. This is obtained by performing a two step transformation process. In the first step the steel is maintained at 350 - 500°C and in the subsequent step the temperature is increased and held thereon until bainite transformation is completely finished. Brief summary of the invention The object of the invention is to provide a method for complete bainite hardening which reduces the bainite transformation time without reducing the hardness. With the new process compared to traditional isothermal bainite transformation it is possible to shorten the process time for a given hardness, or, for a given transformation time, increase the hardness. According to the invention this is achieved with the method of claim 1 by starting the bainite transformation at a temperature just above the Ms temperature of between about 180°C and about 280°C depending on the composition of the steel, transforming 25-99% of the austenite into bainite, and then increasing the temperature to speed up the transformation of the remaining austenite into bainite. According to a preferred embodiment of the invention, about 50-90 % of the austenite is transformed into bainite before increasing the temperature for speeding up the transformation. Brief description of the drawings The invention will be described more in detail below with reference to the accompanying drawings, wherein Fig. 1 is a schematic drawing of two cycles giving approximately the same hardness but one with a significant shorter transformation time, and Fig. 2 is a schematic drawing of two cycles with the same transformation time but one with significantly higher hardness. Description of preferred embodiments of the invention In Fig. 1 the schematic drawing the temperature vs. log time is plotted for two cycles giving approximately the same hardness, i.e. 60 HRC. Ms denotes the martensite start point, which normally varies between about 180° and 280°C, depending on the composition of the steel alloy. Bs denotes the start of bainite transformation and Bf denotes the finish of bainite transformation. In order to obtain maximum hardness the temperature should be close to the martensite start point. This, however, results in a very long transformation time which is not economical, which is illustrated with the dashed line in Fig. 1. According to the invention, illustrated with the solid line in Fig. 1, the bainite transformation is performed slightly above the Ms point until more than 25 % up to 99 %, preferably 50 - 90 % transformation, and the remaining hardening is performed at the temperature at which the transformation proceeds with increased velocity to 100 %. The fraction of bainite formed at the higher temperature has not shown any noticeable effects on the hardness. Another way to use the process according to the invention is to increase the hardness for a given transformation time, which is illustrated in Fig. 2. In the process illustrated with a dashed line, the transformation takes place at the optimum temperature in view of the velocity of transformation, versus time consumption for 100 % transformation to bainite. The resulting material has a hardness of 58 HRC. According to the invention, the bainite hardening is performed along the solid line at a lower temperature to more than 50 %, preferably 60 - 80 % bainite transformation, and then the temperature is increased to the optimum temperature. This results in a steel alloy with a hardness of 60 HRC, using the same transformation time. In practice both features could be achieved compared to traditional bainite hardening, i.e. shorter transformation time and higher hardness. The results also show increased structural stability which is very important in many applications. The invention will be further illustrated in the examples below. Examples The tests were carried out in a dilatometer. The composition of the steel was according to Table 1. Chemical composition in weight%. C Si Mn Cr Mo 0,96 0,09 0,84 1,91 0,56 The example in Table 2 shows that for a transformation time of 9h +1h (No# 1) with the new bainite cycle it is possible to obtain a hardness of approximately 60 HRC. If the same hardness shall be reached for a conventional bainite hardening the transformation time will be approximately 33h (No#2). A transformation time of 10 hours with a conventional bainite cycle requires that one increases the temperature and consequently the hardness will drop to approximately 59 HRC (No#3). Heat treatment experiments No# Aust. T Aust t. Trans f T Trans f t Temp T Temp t Hardness 1 885°C 3h 20min 210°C 9h 250 1h 59,9 HRC 2 885°C 3h 20min 210°C 33h N.A N.A 60,1 HRC 3 870°C 2h 230°C 10h N.A N.A 58,8 HRC N.A. = Not applied

Claims: A method of complete bainite hardening of an austenitized steel for use in bearings and other load carrying components, characterized in that 25 - 99 % of the austenite is transformed into bainite at a temperature just above the martensite formation temperatures of between about 180°C and about 280°C depending on the composition of the steel, and subsequently the temperature is increased in order to speed up the transformation of the remaining austenite into bainite. A method according to claim 1, wherein about 50 - 90 % of the austenite is transformed into bainite before increasing the temperature.

Inventors: LARSSON STAFFAN, LUND THORE, OELUND PATRIK, LARSSON, STAFFAN, LUND, THORE, OELUND, PATRIK, Ölund, Patrik
Applicants: OVAKO STEEL AB
Application year: 1998
Main classification: C21D1
IPC codes: C21D1, C21D6