Effect of Quenching Process on Microstructure and Properties of 110-Grade 37Mn5 Thick-Walled Steel Pipe

Literature Overview

The paper published in Steel Pipe (2020, Vol. 49, No. 5, pp. 61-64) by Wang Jinyong and colleagues from Xinxing Cast Iron Tube Co., Ltd. investigates the influence of quenching strategies on the microstructure and mechanical properties of 110-PSL grade 37Mn5 thick-walled steel pipes. This work is particularly relevant to high-strength line pipe manufacturing where uniform hardness and toughness across the wall thickness are critical for pipeline integrity under high-pressure service conditions. The study compares two quenching approaches: internal spray-only (inner jet) and combined external shower with internal spray (outer shower plus inner jet), and evaluates the effect of micro-alloying adjustments on quenchability and post-tempering microstructure.

Core Technical Findings

The authors systematically examined the relationship between quenching method, chemical composition modification, and the resulting microstructural evolution in 37Mn5 steel, which is a manganese-chromium alloy steel commonly used for high-strength line pipe applications. The key findings are summarized below:

The critical insight from this work is that the combined external shower and internal spray method achieves a more uniform cooling rate across the entire wall thickness of the thick-walled pipe. In thick-walled pipe sections, the outer surface cools significantly faster than the inner surface during internal spray-only quenching, creating a steep thermal gradient that leads to differential microstructural evolution. The external shower provides additional convective and evaporative cooling on the outer surface, while the internal spray ensures adequate cooling of the inner surface, thereby narrowing the thermal gradient and promoting more homogeneous transformation kinetics throughout the cross-section.

Quenchability Enhancement Through Micro-Alloying

The elevation of manganese content from 1.2% to 1.5% and chromium content from 0.15% to 0.25% plays a dual role in this system. Manganese is a potent austenite stabilizer that widens the austenite phase field and reduces the critical quenching rate required for martensitic transformation. Chromium, as a carbide-forming element, contributes to solid solution strengthening and also enhances quenchability by retarding the nose of the TTT (Time-Temperature-Transformation) curve. The combined effect of these two alloying additions, together with the improved cooling uniformity of the dual-spray method, ensures that even the inner surface of the thick-walled pipe achieves sufficient cooling rates to suppress ferrite and pearlite formation, resulting in a predominantly martensitic structure before tempering.

Tempered Sorbite Formation and Property Stability

After quenching, the pipe is subjected to tempering treatment to convert the hard and brittle martensite into tempered sorbite. The study confirms that when the combined quenching method is employed with the elevated alloy content, the tempered sorbite microstructure is both uniform and dense, free from residual ferrite networks that would act as preferential paths for crack initiation and hydrogen-induced cracking. The elimination of the ferrite network is particularly important for pipeline applications where the pipe may be subjected to bending during installation, as ferrite networks reduce ductility and promote intergranular fracture under strain.

Engineering Practice Implications

From a manufacturing standpoint, this study provides actionable guidance for producers of high-strength thick-walled line pipes. The transition from internal-spray-only to combined external shower plus internal spray represents a practical modification to existing quenching lines that does not require fundamental process redesign. However, the installation of external shower nozzles must be carefully designed to ensure uniform water distribution around the pipe circumference and along its length. In practice, nozzle spacing, spray angle, and water pressure must be optimized to avoid localized overcooling that could induce excessive bending or distortion.

The recommendation to increase Mn to 1.5% and Cr to 0.25% also has implications for the upstream hot rolling and annealing processes. Higher manganese content can increase hot shortness susceptibility during rolling, requiring careful control of sulfur and phosphorus levels and appropriate rolling temperature windows. Chromium addition may require adjustments to the reheating temperature to ensure complete dissolution of chromium carbides prior to austenitization.

Process Window Considerations

For thick-walled 37Mn5 pipe in the 110-PSL grade, the typical wall thickness range is 15-25 mm. The critical cooling rate for complete martensitic transformation in this composition is approximately 15-25 °C/s for the inner surface and 40-60 °C/s for the outer surface under combined spray conditions. These values must be verified through Jominy end-quench tests on production heats to establish the precise quenchability of each heat before committing to the quenching cycle. The tempering temperature window is typically 600-650 °C for 2-3 hours, followed by air cooling, to achieve the target hardness of 250-280 HBW while maintaining adequate Charpy V-notch impact energy at the minimum service temperature.

Study Insights and Reflections

This paper exemplifies the principle that in thick-walled pipe manufacturing, the cooling strategy must be designed with explicit consideration of the thermal gradient across the wall thickness. The dual-spray approach is not merely a matter of applying more cooling but rather achieving balanced cooling that respects the different heat capacities and thermal conductivities of the inner and outer surfaces. The synergy between micro-alloying for quenchability enhancement and process modification for cooling uniformity is a textbook example of materials-process co-design.

One area that could benefit from further investigation is the long-term hydrogen-induced cracking resistance of the tempered sorbite produced under these conditions. The elimination of the ferrite network is favorable, but the retained austenite content after quenching and tempering should be quantified, as retained austenite can act as a hydrogen trap and influence the susceptibility to HIC and SSC in sour service environments. Additionally, the effect of cooling water chemistry (hardness, pH, temperature) on the quenching severity should be documented, as these factors can significantly influence the cooling curve and hence the microstructural outcome.

The practical value of this study is high for manufacturers seeking to expand their product range into thick-walled high-strength line pipe, as it provides a clear and implementable pathway to achieve the required mechanical properties with improved consistency and reduced bending distortion.