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

Literature Overview

This paper, published in the journal "Steel Pipe" in 2020 (Volume 49, Issue 5, pages 61-64) by Wang Jinyong and colleagues from Xinxing Ductile Iron Pipe Co., Ltd., investigates the influence of quenching methods on the microstructure and mechanical properties of 110 grade 37Mn5 thick-wall steel pipe. The study is particularly relevant for engineers involved in the production of high-strength line pipe, where the quenching and tempering (Q+T) process window directly determines the final toughness and strength balance.

Core Technical Findings

The authors compared two quenching approaches: internal spraying only versus external rinsing combined with internal spraying. The key conclusion is that increasing the manganese content to 1.5% and chromium content to 0.25%, combined with the external rinsing plus internal spraying method, effectively reduces pipe bending during quenching and improves the stability of mechanical properties.

The metallurgical mechanism is clear: the combined quenching method ensures sufficient austenite-to-martensite transformation throughout the thick wall cross-section, eliminating reticular ferrite networks that would otherwise compromise toughness. After tempering, a uniform and dense tempered sorbite microstructure is achieved.

Process Parameters and Metallurgical Analysis

The thick-wall nature of 37Mn5 pipe presents a classic challenge in Q+T processing. The core of the wall cools more slowly than the outer surface, creating a gradient in cooling rates that can lead to mixed microstructures. The external rinsing step addresses this by ensuring the outer surface does not cool excessively fast, while the internal spray drives cooling at the inner surface, effectively narrowing the cooling rate differential across the wall thickness.

Engineering Practice Implications

From my experience with line pipe production, the elimination of reticular ferrite is critical for meeting impact energy requirements, particularly at the weld heat-affected zone. When reticular ferrite persists after quenching, it acts as a brittle phase that reduces Charpy V-notch energy at low temperatures. The specification of 110 grade (minimum yield strength of 758 MPa) demands a fine tempered sorbite structure with a carbon equivalent controlled to ensure adequate weldability.

The addition of 0.25% chromium to the 37Mn5 base composition enhances hardenability, which is essential for thick-wall pipe where the critical diameter for full hardenability is inherently limited by the manganese-only alloying. Chromium also contributes to elevated temperature strength, making the steel suitable for higher-temperature service conditions.

Key Reflections

This study exemplifies the principle that in thick-wall pipe Q+T processing, the geometry of the cooling circuit is as important as the chemical composition. Engineers must not view quenching as a simple immersion step but as a carefully designed thermal management process. The PDCA cycle applies here: the plan involves selecting the quenching method based on wall thickness and alloy content; the do phase involves process validation with metallographic verification; the check phase compares achieved properties against the technical agreement; and the act phase involves refining the quenching parameters for subsequent heats.

Summary

The research demonstrates that for 110 grade 37Mn5 thick-wall steel pipe, the combination of optimized chemistry (1.5% Mn, 0.25% Cr) and a dual-mode quenching approach (external rinsing plus internal spraying) is essential to achieve full martensitic transformation, eliminate reticular ferrite, and obtain a uniform tempered sorbite microstructure that meets the stringent mechanical property requirements. This finding has direct practical significance for manufacturers aiming to produce high-strength line pipe with reliable toughness and dimensional stability.