Microstructure and Properties of L360Q Pipeline Steel Hot Induction Bends: Effects of Tempering Heat Treatment

Using a hot induction bending test, straight seamless L360Q steel pipes were locally heated by induction, bent, and then tempered. The hardness, tensile properties, and impact toughness of different regions of the pipe bend were systematically evaluated, and their microstructures were characterized. In addition, the mechanical properties of the L360Q steel were compared before and after hot induction bending and heat treatment. The test results indicate that the non-uniform temperature distribution during heating caused different regions of the pipe bend to undergo varying degrees of deformation, resulting in corresponding variations in microstructure and mechanical properties. The arc-start region of the transition zone exhibited a tensile strength of 588 MPa and a yield strength of 520 MPa, both of which were significantly higher than those of the original base metal (straight seamless pipe) and the other regions of the pipe bend. The hardness values in the regions near the arc-start and arc-end sections were higher than those in the remaining pipe body. Although the elongation of the base metal decreased slightly after hot bending and tempering, the overall toughness remained largely unchanged, indicating that these processes had a limited effect on the toughness of the steel pipe. The hot bending and heat treatment processes employed in this study comply with the requirements of GB/T 9711—2023 and SY/T 5257—2012, providing a technical foundation and practical reference for the design and manufacturing of L360Q pipeline bends.

With the rapid advancement of industrialization, pipeline transportation has become increasingly important in sectors such as oil, natural gas, and electric power, serving as a critical infrastructure component that supports their continued development. Regarding pipeline alignment and directional change technologies, approaches such as elastic laying, prefabricated elbow installation, and cold bending are subject to various practical limitations. In contrast, hot induction bends, supported by advanced and well-established manufacturing technologies and offering stable and reliable quality, are gradually gaining a dominant position in the industry and becoming the preferred solution for many engineering applications. Hot-induction bending is particularly advantageous for manufacturing bends with various required angles, as it enables continuous production through a dedicated production line while offering high flexibility and adaptability. These characteristics allow it to effectively meet the evolving requirements of oil and gas pipeline construction. However, the hot-induction bending process still faces several technical challenges, including quality defects such as uneven deformation, surface cracking, internal voids, and non-uniform wall thickness. Taking the production of L360-grade bends as an example, significant microstructural changes occurring during the manufacturing process can result in excessive increases in hardness, tensile strength, and yield strength, accompanied by reductions in impact toughness and elongation after fracture. Consequently, the overall mechanical properties may fail to satisfy the relevant standard requirements, thereby compromising the service performance and operational safety of the product. To address these issues, this study investigates the hot-bending process of L360Q steel through experimental research. It investigates the microstructure and performance characteristics of L360Q steel bends in depth, aiming to provide reliable practical guidance for industrial-scale production and to support the development of the pipeline manufacturing industry through systematic experimental analysis and data synthesis. This study provides an in-depth investigation of the microstructure and performance characteristics of L360Q steel bends, aiming to offer reliable practical guidance for industrial-scale production and support the advancement of pipeline manufacturing through systematic experimental analysis and data integration.

The material used in this study was a straight seamless L360Q steel pipe with dimensions of φ168.3 mm × 10 mm. The manufacturing process consisted of the following steps: inspection and acceptance of the straight seamless pipe, hot bending, end rounding, end beveling, tempering, sandblasting and grinding, dimensional inspection, non-destructive testing, secondary sandblasting, and final storage. The straight sections of the hot-pipe bend were 300 mm in length, with a bending radius of R = 5D (five times the pipe diameter) and a bending angle of α = 33°. The chemical composition of the test bends is presented in Table 1. Tempering was carried out using a 5 m heat treatment furnace with an effective internal chamber size of 5 m × 2.5 m. The furnace employed natural gas heating and was equipped with a PLC-based automatic control system to precisely regulate the heating rate, holding temperature, and holding time. The furnace exhibits high temperature uniformity, with a deviation of no more than 5°C, thereby ensuring the consistency of material heat treatment. Throughout the heat treatment process, the effective furnace temperature is maintained within a range of ±10°C.

Table 1. Chemical composition (mass fraction, %) of the L360Q steel used in the experiment

After the straight seamless steel pipe was cut to the required length, it was formed using a push-bending machine. An induction coil was positioned around the section of the pipe to be bent, while a mechanical rotating arm securely clamped the pipe end to facilitate the bending process. The steel pipe was heated by passing a medium-frequency current through the induction coil. Once the temperature of the L360Q steel pipe reached 900°C and the material entered the plastic deformation stage, a mechanical thrust force was applied to the pipe end, driving the pipe forward at a rate of 50 mm/min. After bending, the deformed section of the pipe was rapidly cooled with water. The entire process was carried out continuously, with heating, pipe advancement, bending, and cooling occurring simultaneously until the desired bend geometry was achieved (Figure 1). The hot-bent L360Q steel pipe with dimensions of φ168.3 mm × 10 mm was subjected to tempering treatment in a heat treatment furnace. The tempering process was conducted at 600°C for 30 min, followed by furnace cooling and then air cooling to room temperature after removal from the furnace. The heating and cooling rates were uncontrolled below 400°C, while they were maintained below 200°C/h above 400°C. After tempering, the small-diameter induction-pipe bend was subjected to hardness testing, tensile testing, Charpy impact testing, and metallographic examination. Specimens were collected from the locations shown in Figure 2.

Figure 1. Hot induction bending process

Figure 2. Sampling locations on the pipe bend

1 to 8:

• Upper region of the straight pipe section
• Outer arc side of the transition zone at the bend start
• Outer arc side of the bent section
• Upper region of the neutral zone in the bent section (where wall thickness remains essentially unchanged)
• Outer arc side of the transition zone at the bend end
• Lower region of the neutral zone in the bent section (where wall thickness remains essentially unchanged)
• Inner arc side of the bent section
• Lower region of the straight pipe section

According to the technical requirements specified in GB/T 9711—2023 Petroleum and natural gas industries — Steel pipe for pipeline transportation systems, the seamless steel pipe investigated in this study was manufactured from L360Q steel. During the bending process, the heating temperature reached 900°C, followed by external water cooling and subsequent tempering. Due to the significant variation in properties among different regions of the pipe after hot bending, stress-relief annealing was conducted at 590°C to reduce residual stresses. The pipe bend was subjected to heat treatment at 600°C, after which the microstructure and grain size were examined at various locations, including the bent section (inner arc side, outer arc side, and neutral zone where wall thickness remained essentially unchanged) and the start and end regions of the transition zone. As shown in Figure 3, the microstructures in the transition zones at the bend start (induction heating initiation region) and bend end (bend completion region) exhibit banded features. The microstructural constituents at the inner surface, outer surface, and mid-wall thickness of both transition zones are mainly composed of pearlite, ferrite, and bainite. This is attributed to the fact that the arc-initiation and arc-termination points, which serve as the starting and ending regions of the heating–cooling cycle, experience higher thermal and mechanical stresses. These conditions accelerate deformation-induced austenite transformation and promote the formation of granular bainite. Figure 4 illustrates the microstructural morphology of the outer and inner regions of the curved section of the pipe bend. As shown in Figure 4, the microstructure exhibits a banded morphology and is primarily composed of pearlite and ferrite. At the outer arc side, tensile stresses cause wall thinning and accelerate cooling. Nevertheless, the concentrated and stable heat input promotes complete austenitization and subsequent microstructural homogenization during heat treatment, which inhibits the formation of granular bainite. In contrast, the inner arc side experiences compressive stress, causing wall thickening and a slower cooling rate. Owing to the relatively limited deformation and inadequate driving force for phase transformation, the microstructure remains dominated by equilibrium phases, consisting mainly of pearlite and ferrite.

a) Inner surface of the transition zone arc-initiation section

b) Outer surface of the transition zone arc-initiation section

c) Mid-wall thickness of the transition zone arc-initiation section

d) Inner surface of the transition zone arc-termination section

e) Outer surface of the transition zone arc-termination section

f) Mid-wall thickness of the transition zone arc-termination section

Figure 3 Metallographic structure of the transition zone

a) Inner surface of the curved segment (inner arc side)

b) Outer surface of the curved segment (inner arc side)

c) Mid-wall thickness of the curved segment (inner arc side)

d) Inner surface of the curved segment (outer arc side)

e) Outer surface of the curved segment (outer arc side)

f) Mid-wall thickness of the curved segment (outer arc side)

Figure 4 Microstructure of the inner and outer arc sides of the bent section

Figure 5 presents the microstructures of the neutral zone in the bent section, where the wall thickness is essentially unchanged. The microstructures at the inner surface, outer surface, and mid-wall thickness regions display distinct banded characteristics, with pearlite and ferrite as the predominant phases.

a) Inner surface of the neutral zone  b) Outer surface of the neutral zone  c) Mid-wall thickness of the neutral zone

Figure 5 Microstructure of the neutral zone in the bent section

A detailed examination of the grain size in the transition zones, bent section, and neutral zone revealed that the prior austenite grain size in all these regions was classified as Grade 10. The obtained grain size is significantly finer than the Grade 7 requirement defined by the technical standards, demonstrating the effectiveness of the manufacturing process in promoting grain refinement and enhancing material properties while satisfying or surpassing the required specifications.

Following the tempering treatment, systematic hardness measurements were performed on critical regions of the pipe bend. These regions included the bent section, consisting of the inner arc side, outer arc side, and neutral zone (where wall thickness remains essentially unchanged), as well as the transition zones at the bend start and bend end. The test results showed that the hardness values of all measured regions were below 210 HV10, satisfying the hardness requirements for L360Q steel specified in GB/T 9711—2023 and confirming that the hot-induction pipe bend meets the required hardness specifications. As shown in Figure 6, the hardness values in the arc-start and arc-end transition zones were significantly higher than those in the main body of the bent section. This can be attributed to the contribution of M/A islands within the granular bainite and the high density of dislocations, both of which enhance the hardness of the material. Furthermore, repeated thermal cycling promoted the refinement of precipitated phases, while deformation at higher strain rates intensified work hardening, further contributing to the increase in hardness.

In accordance with the requirements of GB/T 9711—2023 and GB/T 228.1—2021 Metallic materials — Tensile testing — Part 1: Method of test at room temperature, tensile tests were conducted using a tensile testing machine on specimens extracted from the original straight seamless steel pipe and various regions of the pipe bend. The mechanical properties of the pipe bend satisfy the standard requirements for L360Q steel, with a specified tensile strength range of 460–760 MPa and a yield strength range of 360–530 MPa. Specifically, the arc-initiation section of the transition zone exhibited a tensile strength of 588 MPa and a yield strength of 520 MPa, both of which were significantly higher than those of the original base metal (straight seamless pipe) and other regions of the pipe bend. This can be attributed to the formation of granular bainite in the arc-initiation section during repeated thermal cycles, where the hard phase contributes to the enhancement of material strength. Furthermore, the arc-initiation section experienced high stress and rapid cooling during the hot-bending process, resulting in an increased dislocation density that further strengthened the material.

The original straight seamless steel pipe exhibited an elongation of 46%, which was significantly higher than that of the various sections of the pipe bend. This indicates that the plasticity of L360Q steel decreased after hot bending and tempering treatments. In accordance with the requirements of GB/T 9711—2023 and GB/T 228.1—2021, Charpy impact toughness tests were conducted at −20°C using an impact testing machine on specimens (7.5 mm × 10 mm × 55 mm) extracted from various regions of the pipe bend. The impact energy values of the straight pipe body, pipe bend body (including the thickened zone on the inner arc side, thinned zone on the outer arc side, and neutral zone where wall thickness remains essentially unchanged), arc-initiation section of the transition zone, and arc-termination section of the transition zone all satisfy the requirements specified in SY/T 5257—2012 Induction-heated steel pipe bends for oil and gas transportation.

The standard specifies that the average impact absorption energy of three specimens shall be no less than 90 J. The impact absorption energy values of the straight pipe section, arc-end transition zone, inner arc side, and neutral zone of the bend were comparable and were all higher than those of the arc-start transition zone and the outer arc side. This can be attributed to the formation of granular bainite in the arc-start transition zone and the outer arc side of the bend. The combined effects of mixed-grain microstructures and high residual stresses reduced the toughness of these regions. In contrast, the inner arc side, neutral zone, and straight section exhibited higher toughness due to their relatively uniform microstructures, refined grains, and lower residual stress levels.

1) The performance of the Φ168.3 mm × 10 mm hot-induction pipe bend was verified through comprehensive testing. The results demonstrated compliance with the technical requirements specified in GB/T 9711—2023 and SY/T 5257—2012, confirming the reliability of the manufacturing process and providing a technical basis and reference for the design and production of L360Q pipeline bends.

2）Microstructural characteristics of the bend transition zone: The transition zone exhibited complex microstructural features. At the inner surface, outer surface, and mid-thickness regions of both the arc-start and arc-end transition zones, the microstructure consisted mainly of pearlite, ferrite, and granular bainite.

In contrast, the microstructure of the bent section was primarily composed of pearlite and ferrite. This indicates that the transition zone possesses a more complex microstructure than the bent section and is more significantly affected by non-uniform temperature distribution.

3）Changes in elongation of seamless steel pipe after hot bending: After hot bending and tempering, the elongation of the seamless steel pipe decreased slightly; however, no significant overall reduction in toughness was observed. This indicates that the hot-bending and tempering processes had a limited effect on the toughness of the pipe.

4) Influence of heating non-uniformity on L360Q pipe bends: During the heating process, the non-uniform temperature distribution in L360Q pipe bends causes different degrees of deformation among various regions, leading to variations in microstructure and properties. The transition zones near the induction heating start and end points exhibit significantly higher hardness values than the main bent section.

Furthermore, the tensile strength and yield strength of the start-point transition zone are significantly higher than those of the original base material (straight seamless steel pipe) and other regions of the bend.