Influence of Long-Term Subcritical Annealing on the Unalloyed Steel Welded Joint Microstructure

The article presents changes in the microstructure of hot-rolled unalloyed structural steel after the arc welding process and in the state after long-term exposure to 600 °C during operation. These studies enable a clear assessment of the effects of long-term exposure to elevated temperature relative to the as-welded condition, which has not been reported. The microstructure examination was carried out on welded joints in eight different zones of the joint. Studies have shown that the welding thermal cycle causes significant changes in the microstructure in the area of the base material heated above the A 1 temperature—the heat-affected zone (HAZ)—and in the weld area in the case of multi-pass welding. The long-term exposure of the subcritical temperature of 600 °C on the welded joint leads to the phenomenon of cementite spheroidization in the pearlite in all zones of the joint, while preserving the band structure of the steel after rolling and the structural structure. In the case of the weld, acicular and side-plate ferrite disappearance was observed.

1. Introduction

Structural steels are used in many industries, from everyday products to large building structures. Due to their uncomplicated chemical composition and the use of conventional manufacturing processes for their production, they are characterized by good processing properties—deformability and weldability [1,2,3,4,5].

The banded ferritic-pearlitic microstructure of the steel obtained in the hot-rolled condition undergoes some controlled and uncontrolled changes due to thermal and mechanical influence [6,7,8]. In the case of inseparable joining of elements in welding processes, a new component is created that connects two materials—a weld—with a dendritic structure, the characteristics of which are similar to those of the cast material. The material joined in the weld area is characterized by changes caused by rapid heating to a temperature lower than solidus and subsequent cooling, which leads to the formation of a heat-affected zone. As they move away from the fusion line, individual areas are heated to lower and lower temperatures. Despite the relatively short time of staying at a given maximum temperature and fairly quick cooling, areas identical to those after ordinary heat treatment—annealing with phase transformation—are observed in the HAZ (heat-affected zone) of structural steels. The highest heated area—CGHAZ (coarse grain HAZ)—is characterized by a coarse-grained structure of the former austenite grain, similarly to that after homogenizing annealing. The area heated slightly above the temperature of the end of the eutectoid transformation—FGHAZ (fine grain HAZ)—is characterized by a fine-grained structure, similar to normalized steel. In the area heated in the range of intercritical temperatures A1 (which defining the lower limit of existence of austenite) and A3 (which defining the upper limit of existence of ferrite)—ICHAZ (intercritical HAZ)—the material undergoes partial grain refinement, i.e., in the area that has undergone recrystallization, similar to during incomplete annealing. Areas of hot-rolled steel heated to a subcritical temperature do not undergo any visible transformation [9,10,11], unlike martensitic steels, where an area softened due to tempering (over-tempered region) is observed [12].

Welded joints, due to changes in the microstructure or state of stress, are subjected to heat treatment, including annealing processes with transformation (e.g., normalization) and without transformation (stress relief annealing) [13,14]. The impact of elevated temperature may also be related to the operating conditions of welded structures, which mainly depends on their application. There are needs, such as boilers or reactors, in which the increased process temperature, below the critical temperature, affects the structure in a way analogous to the heat treatment without transformation [15,16]. An example of such a process is lead refining, which, depending on the type of pollutant to be removed, is carried out in various temperature ranges, even reaching 600 °C [17]. It can be expected that long-term operation in this temperature range will give effects that are observed, for example, after spheroidizing annealing.

ISO 4885 [18] defines spheroidizing as annealing just below the A1 temperature of steels with long soaking time to bring the carbides in the form of spheroids. The microstructure obtained in this process is spheroidite, defined as characteristic soft microstructure consisting of sphere-like globular cementite particles within a ferrite matrix. Wang et al. [19] presented a diagram of the progress of the spheroidization process of lamellar cementite in pearlite ( ).

Although the ISO 4885 standard and other textbook heat treatment charts indicate a temperature range of spheroidizing annealing oscillating around the A1 temperature, there are a number of studies proving that this process also occurs at lower temperatures. Stodolny et al. [20] found a 100% content of spheroidite in C45 steel after annealing at 700 °C/1 h and 600 °C/23 h, and at 500 °C after 23 h an increase in the content of spheroidite of 11% was achieved. Wang et al. [19] conducted spheroidization in 14Cr1MoR steel at 680 °C for 22, 40, 70, and 100 h, observing the continuous progress of spheroidization, represented, among others, by a decrease in hardness from approximately 32.5 HRC to approximately 24 HRC after 100 h of annealing. Yang and Lu [21] observed a decrease in the mechanical properties of cold-rolled SCM435 steel after spheroidization at 700 °C/5 h and 680 °C/5 h, however, the observed effects are related to both carbide spheroidization and recrystallization of cold-deformed grains. Arruabarrena and Rodriguez-Ibabe [22] found a decrease in the hardness of fine pearlite with increasing annealing time at 720 °C, 660 °C, and 600 °C in AISI 5140 steel. On the basis of the studies mentioned above, it can be assumed that a very long exposure to temperatures below A1 may, in consequence, also lead to spheroidization of cementite, similar to during annealing around the A1 temperature.

As part of the research, the microstructure of the welded joint of unalloy steel in the as-welded condition and in the condition after long-term exposure at 600 °C was analysed. Long-term exposure (amounting to over 4000 h) is related to the operation of the product that contains the analysed welded joint at a temperature of 600 °C. The aim of the research is to assess the influence of the state of welded joints on their microstructures. These studies enable a clear assessment of the effects of long-term exposure to elevated temperature relative to the as-welded condition, which has not been reported.