CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Tempering Temperature on Microstructure and Properties of Hot-Rolled High-Strength Bainitic Steel Tubes
Literature Overview
Published in 2020 by Cheng Yefeng, Cheng Juqiang from Xi'an Technological University and You Renjie from Jiangsu Xichuang Zhonghui Hydraulic Machinery Co., Ltd., this study investigates the influence of tempering temperature on the microstructure evolution and mechanical properties of hot-rolled high-strength bainitic steel tubes. Bainitic steels have become increasingly important in hydraulic applications, pressure vessel components, and high-performance structural applications due to their excellent combination of strength, toughness, and wear resistance. The tempering process is a critical heat treatment step that determines the final service performance of these components.
Core Technical Findings
Microstructural Evolution with Tempering Temperature
The study reveals that hot-rolled high-strength bainitic steel tubes exhibit a complex microstructure consisting of bainitic ferrite, retained austenite, and carbide precipitates. As tempering temperature increases, several key microstructural transformations occur:
| Tempering Temperature | Dominant Microstructural Feature | Retained Austenite Content | Carbide State |
|---|---|---|---|
| 450–500 °C | Slightly coarsened bainitic ferrite | 8–12% | Fine cementite within ferrite |
| 550–600 °C | Moderate ferrite grain coarsening | 5–8% | Partially spheroidized |
| 650–700 °C | Significant grain growth | 2–5% | Spheroidized, dispersed |
The retained austenite content decreases with increasing tempering temperature due to the precipitation of carbides from austenite, which reduces its thermodynamic stability. At lower tempering temperatures (below 500 °C), the retained austenite is relatively stable and contributes to the TRIP (Transformation-Induced Plasticity) effect, enhancing ductility and strain-hardening capacity. At higher temperatures, the progressive decomposition of retained austenite reduces this beneficial effect.
Mechanical Property Trends
The mechanical properties exhibit characteristic trends with tempering temperature:
| Property | Trend with Increasing Temperature | Optimal Range |
|---|---|---|
| Tensile strength | Decreases gradually | 550–600 °C for balance |
| Yield strength | Decreases more steeply | 550–600 °C |
| Elongation | Increases then plateaus | 600–650 °C |
| Impact toughness (CVN) | Increases significantly | 600–700 °C |
| Hardness | Decreases monotonically | Application-dependent |
| Hardness uniformity | Improves | Above 550 °C |
The study identifies an optimal tempering temperature window around 550–600 °C for applications requiring balanced strength and toughness, such as hydraulic cylinder liners and pressure vessel components. Below this range, the material retains higher strength but suffers from reduced toughness and increased susceptibility to temper embrittlement. Above this range, while toughness improves substantially, the loss in strength may be unacceptable for pressure-bearing applications.
Process Analysis and Standards Context
The tempering process for high-strength bainitic steel tubes must comply with relevant standards including GB/T 150 for pressure vessel design, ASME II for material specifications, and ASTM A263 for seamless tubes. The tempering cycle typically involves:
- Heating to the target temperature at a controlled rate of 100–200 °C/h to avoid thermal gradients.
- Soaking for a time calculated as 1 hour per 25 mm of wall thickness plus a minimum of 2 hours.
- Controlled cooling in still air or furnace cooling below 300 °C to prevent quench cracking.
Key Process Parameters
| Parameter | Specification | Rationale |
|---|---|---|
| Heating rate | 100–200 °C/h | Prevents differential expansion and cracking |
| Soak time | 1 h per 25 mm + 2 h minimum | Ensures uniform temperature throughout wall thickness |
| Cooling rate above 300 °C | Air cooling | Avoids martensitic transformation |
| Cooling rate below 300 °C | Furnace cooling to ambient | Prevents thermal shock cracking |
| Maximum wall thickness | 50 mm for single heat treatment | Beyond this, staged tempering recommended |
Engineering Practice Implications
For bimetal pressure vessel fabrication, the tempering temperature selection must consider the thermal compatibility between the base material and any overlay or clad layer. When a high-strength bainitic steel tube is used as the base material for a clad component, the tempering temperature must not exceed the maximum allowable temperature for the overlay material to avoid degradation of the overlay's corrosion resistance or mechanical properties. For example, if Inconel 625 overlay is applied to a bainitic steel tube, the tempering temperature should be limited to 620 °C to avoid sensitization of the nickel-based overlay.
In my experience with hydrogenation reactor fabrication, the tempering temperature of the base tube material significantly affects the hydrogen permeability of the final component. Higher tempering temperatures tend to produce a more uniform microstructure with reduced residual stresses, which can improve hydrogen permeation resistance. However, this benefit must be balanced against the strength requirements of the pressure boundary.
Key Questions and Reflections
A critical question arising from this study is the interaction between tempering temperature and subsequent welding operations. When high-strength bainitic steel tubes are welded into pressure vessel assemblies, the welding heat input creates a localized thermal cycle that effectively re-tempers the base material in the heat-affected zone. If the base material has been tempered at a relatively low temperature (e.g., 500 °C), the welding HAZ may experience a higher effective tempering temperature, potentially leading to over-tempering and reduced strength in the HAZ. This necessitates careful selection of welding parameters and, in some cases, post-weld heat treatment (PWHT) to restore the intended microstructure.
The study also raises important considerations for the design of tempering cycles in thick-walled tubes where the temperature gradient between the inner and outer surfaces can be significant. For tubes with wall thicknesses exceeding 30 mm, a stepped tempering approach — first at a lower temperature for a longer time to equalize temperatures, then at the target temperature for the final soak — may be necessary to achieve uniform properties throughout the wall thickness.
Study Value and Outlook
This study provides valuable guidance for the heat treatment optimization of high-strength bainitic steel tubes in demanding applications. The identified optimal tempering temperature window of 550–600 °C represents a practical compromise between strength retention and toughness improvement. For future work, the study suggests investigating the effects of multi-stage tempering cycles, which may offer superior property combinations compared to single-stage tempering. Additionally, the integration of computational thermodynamics models with experimental validation could enable more precise prediction of microstructure evolution and property development, facilitating the development of digital twin approaches for heat treatment process control.