The effect of heat treatment on the hardness distribution of low carbon steel wires after wire drawing and static strain aging

یاشار خالدزاده، محسن کاظمی نژاد، علی کریمی طاهری

Abstract

In this research, the static strain aging behavior of low carbon steel was investigated after the wire drawing process. For this purpose, the wires were cooled at temperatures of 850, 950, 1000 degrees Celsius and at different speeds (in the furnace, air, and in front of the fan) and then subjected to the wire stretching process. The resulting wires were aged at a temperature of 200 degrees Celsius and a time of 1 minute. The distribution of hardness in the cross section of the wires before and after the aging process was determined by the microhardness measurement method. The results show that with the increase of sleeve temperature, the increase of hardness with sensitivity to strain aging increases and with the increase of cooling rate, the increase of hardness with sensitivity to strain aging decreases. In addition, before the static strain aging process, the hardness distribution in the cross-section of the wire from the center to the surface is upward, and after aging, the hardness distribution is downward. Also, the increase in hardness due to aging is decreasing from the center to the surface.

Introduction

The process of hot rolling wire is one of of the primary forming processes, the Products of which are used in vari ous industries , especial ly wire drawing .. It is obvious that the ability to change the shape and not break the wire dur ing wire stretching and reducing non- uniformity in the cross- section of the produced wires is one of the important demand s of the steel wire drawing workshops , on which the different parameters of the hot rolling line have a significant impact.. Among the parameters of hot rolling, two factors, the end temperature of rolling ((sleeve temperature)) and the cooling rate after hot rolling , which change the final structure and mechanical properties of the product by influencing the conditions of metallurgical phase change , are considered in this research . By increasing. the austenitizing temperature , the reduced hardness locking parameter is easily performed as a result of the austenite grain growth .. It has been shown that the final rolling temperature and austenite grain size are directly proportional to the final ferrite grain size.

At higher cooling rates , the phase change of austenite to ferrite is reduced, and since at low temperatures the germination rate is higher than the grain growth rate, the size of the phase changed ferrite grains is reduced. These changes. increase the strength and decrease the flexibil ity of ste el .

Wire drawing is a process in which the cross- sectional area of ​​the wire, passing through the converging die, is reduced by a tensile force introduced from the outside of the die.. This process is widely used in the industry due to the creation of suitable mechanical properties, suitable surface finish and high dimensional accuracy in the product.. The main parameters of the stretching process are the angle (0)of the mold (0), the reduction of the cross- sectional area (R) and the friction between the part and the mold.. Wire stretching is a process that takes place at very high speeds, and the speed of the wire passing through the mold may reach 200. This high speed,. the friction between the part and the mold, as well as the heat of the deformation increase the temperature of the wire in the drawing process. This increase. in temperature may be 200 psd. Wire drawingis a continuous process and several dies are used in succession to reduce the cross- sectional area of ​​the wire, i.e. the wire is wrapped around the drum after exiting one die and then enters the next die. and this time may reach one minute. Therefore, due to the presence of strain in the wire and its temperature increase,. and since it has been proven that 1 minute is enough for aging, static strain aging occurs in the wire. After 1 the metal is subjected to initial pre-strain(cold working with wire tension), the physical and mechanical. properties of the metal change due to the reactions (of point defects such as dissolved) interstitial atoms with dislocations caused by deformation. The phenomenon is known as strain aging. Among the important factors of wire aging that have been investigated in this research, we can mention the amount of pre- strain and intermediate elements and heat treatment. In his. research, Rashid showed that the low yield stress increases with the increase of the pre- strain value, so that the maximum yield stress changes (57) are observed. at low pre- strains, and after this pre- strain value, the load decreases to a constant value . (57) is reached. Rashid’s ( research) results are shown in Figure 1 for low carbon steel and type s of high strength and low alloy steel s . . .. (.) . .. . . . . . 1 . . . . . . . . . . . . . . .. ..

One of the main elements causing strain aging in steel are carbon and nitrogen dissolved in the field. With the increase of soluble interstitial elements in the field, their influence on the dislocations becomes faster, and as a result, more changes in mechanical properties are observed in shorter times, and the sensitivity to strain aging increases. Due to the increase in the amount of nitrogen and carbon dissolved in steel due to the increase in the temperature of the sleeve with the temperature after the final stage of rolling at a constant cooling rate, the sensitivity to strain aging of the steel increases. On the other hand, as the austenite temperature of coarser austenite grains increases, the grain boundaries decrease and the rate of penetration of interstitial elements (through the boundaries) also decreases and the sensitivity to strain aging decreases. Slow hardening rates of steel from the austenitic temperature help to form carbide and nitride deposits, as a result of which the dissolved carbon and nitrogen elements in the field are reduced and the susceptibility to strain aging is reduced. The cooling rate from the austenitic temperature can increase the sensitivity to strain for two reasons. Increasing the amount of dissolved nitrogen and carbon in steel, increasing the rate of germination of ferrite from austenite and creating a fine grain structure, as a result of which the area of the grain boundary increases and the rate of penetration of interstitial elements through the boundaries increases, and the sensitivity to strain aging increases.