Research on failure mechanism is essential for the prolonging of gun barrel lifetime. To explore the gun barrel failure mechanism, the damage characteristics of a machine gun barrel were characterized. The results show that the failure of the gun barrel is correlated with the peeling of the Cr layer on the bore surface and the softening of the bore matrix. The damage of the Cr layer varies along the axial direction. From the gun tail to the muzzle, the damage mode of the Cr layer changes from peeling to wearing. The damage rate in both the tail and the muzzle is higher than that in the middle of the barrel. The matrix close to the bore surface is softened due to the high temperatures caused by stress–relief–annealing and shooting. The gun tail suffers from higher temperatures, thus being softened more seriously than the other parts. The softened matrix results in an increase in the tendency of plastic deformation of the bore surface and an increment in the bore diameter, which leads to a decrease in the firing accuracy.

The transformation of carbides in a 1.9Cr-1.4Mo-0.3 V secondary hardening steel that was subjected to over-ageing at 600 °C–700 °C has been investigated. The carbides were characterized using scanning electron microscope (SEM), x-ray diffraction (XRD), inductively coupled plasma-atomic emission spectrometry (ICP-AES), and transmission electron microscopy (TEM) preformed on carbon replicas. The results indicate that MC, M₂C, and M₃C were formed during over-ageing from 600 to 700 °C, whereas M₇C₃, and M₂₃C₆ started to be formed at 650 and 700 °C, respectively. In addition, the co-existence of hexagonal and orthorhombic M₇C₃ structures in a carbide particle was firstly observed. M₃C was transformed to other carbides, and the formation of both M₂C and M₂₃C₆ may follow the ‘separate nucleation’ mechanism, whereas M₇C₃ was transformed from M₃C via the ‘in situ nucleation’ mechanism. The crystallographic orientation relationships between the in situ transformed M₇C₃ and M₃C are ( 11 2 ¯ ) M 3 C / / ( 3 3 ¯ 0 1 ¯ ) M 7 C 3 and [ 312 ] M 3 C / / [ 10 1 ¯ 3 ] M 7 C 3 .

The effects of cold rolling reduction rates and recrystallization annealing temperature on the microstructure, texture, and anisotropic properties of high-strength low-alloy (HSLA) steel were investigated using scanning electron microscopy and electron backscatter diffraction. The results revealed that the constituents of recrystallized, substructured, and deformed structures were strongly affected by cold rolling reduction rates ranging from 33.3% to 66.7% and recrystallization temperatures ranging from 780 to 840 °C. At an annealing temperature of 820 °C, when the cold rolling reduction rate was 33.3%, HSLA steel exhibited a low percentage of recrystallization, with cubic, γ-linear, rolled, and Z-texture (the texture at Euler angles φ1 = 30° and Φ = 20°–30°) structures. The rolled texture and Z-texture increased the strength anisotropy and disappeared at high cold rolling reduction rates. When the annealing temperature was increased from 780 °C to 820 °C, the proportion of recrystallized grains increased, the rolling texture disappeared, and grain orientation gradually gathered in the cubic texture and γ line texture, resulting in low anisotropy of strength. At an annealing temperature of 840 °C, the deformation of the grain disappeared; however, the anisotropy increased compared to annealing at 820 °C because of the formation of a new texture of {001}<−1–20>.