Annealing Softening Mechanism of Double-Layer Rolled-Welded Steel Pipes

Literature Overview

This paper by Cheng Zhenbang, Lu Jun, Wang Jike, and Wei Xicheng (2014), published in Heat Treatment of Metals (Vol. 39, No. 6, pp. 71–76), investigates the annealing softening mechanism of double-layer rolled-welded steel pipes using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The research was conducted in collaboration between Wanxi University of Engineering Science, Shanghai University, and Anhui Wuhu Yuanda Pipe Industry Co., Ltd. The paper is classified under TG156.2 (heat treatment of steels).

Core Technical Content

Double-layer rolled-welded steel pipes are manufactured by rolling two layers of steel strip and welding them together, producing pipes with specific mechanical properties and microstructural characteristics. After manufacturing, these pipes often require annealing to achieve desired mechanical properties, particularly improved ductility and reduced strength. The annealing softening mechanism is governed by the evolution of precipitates, dislocations, and the matrix microstructure during the heat treatment process.

The authors studied the microstructure, precipitate phases, and dislocation structures before and after high-temperature annealing to elucidate the fundamental mechanisms responsible for the observed softening.

Microstructural and Mechanical Properties

Property	Before Annealing	After Annealing	Change
Yield strength (Rp0.2)	>200 MPa	<150 MPa	Significant reduction
Elongation (A50)	<30%	>40%	Substantial improvement
Precipitate size	0.05–2 μm	0.05–2 μm	Size range unchanged
Precipitate distribution	Sparse, intragranular	Sparse, intragranular	Distribution maintained
Dislocation density	High	Significantly reduced	Primary softening mechanism

Precipitate Phase Analysis

The precipitate phases identified in both the pre- and post-annealing conditions were primarily:

TiN (Titanium nitride)
Ti(CN) (Titanium carbonitride)
TiS (Titanium sulfide)
Ti₄C₂S₂ (Titanium carbosulfide)
Fe₃C (Cementite)

These precipitates were predominantly intragranular (within the ferrite grains) with sizes ranging from 0.05 to 2 μm and a sparse distribution. The key transformation observed during annealing was the conversion of TiS to Ti₄C₂S₂, which consumed interstitial atoms (carbon and sulfur) from the ferrite matrix, effectively purifying the matrix.

Softening Mechanism

The annealing softening of double-layer rolled-welded steel pipes is attributed to two primary mechanisms:

Matrix purification through precipitate transformation: The transformation of TiS to Ti₄C₂S₂ during annealing removes interstitial atoms from the ferrite matrix. This reduces solid solution strengthening and allows for easier dislocation motion, contributing to the overall softening.
Dislocation density reduction through recovery and recrystallization: The precipitate phases in the matrix promote recovery and recrystallization processes during annealing. These thermally activated processes reduce the dislocation density in the ferrite matrix, which is the fundamental cause of the observed strength reduction and ductility improvement.

Microstructural Evolution During Annealing

Stage	Microstructural Change	Mechanical Effect
Initial heating	Precipitate transformation (TiS → Ti₄C₂S₂)	Matrix purification begins
Recovery	Dislocation rearrangement and annihilation	Reduced residual stress
Recrystallization	New strain-free grains nucleate and grow	Significant strength reduction
Grain growth	Grain coarsening	Further ductility improvement

Engineering Practice Implications

The annealing process for double-layer rolled-welded steel pipes should be carefully controlled to ensure complete dislocation recovery and matrix purification. Insufficient annealing may result in retained strength and inadequate ductility.
The presence of Ti-bearing precipitates is beneficial for the annealing softening process, as they promote recovery and recrystallization. Steel grades with appropriate Ti content should be selected for applications requiring post-annealing softening.
The transformation of TiS to Ti₄C₂S₂ is a critical process that should be monitored during annealing. This transformation requires sufficient temperature and time to be complete, and incomplete transformation may result in residual matrix strengthening.
The dislocation density after annealing should be verified through mechanical testing (yield strength and elongation) to ensure that the target properties are achieved.
For quality control, metallographic examination and TEM analysis can be used to verify the microstructural state after annealing, particularly the precipitate distribution and dislocation density.

Study Insights

This study provides valuable insights into the metallurgical mechanisms governing the annealing behavior of double-layer rolled-welded steel pipes. The identification of the TiS to Ti₄C₂S₂ transformation as a key process in matrix purification is particularly significant, as it links the precipitate chemistry to the macroscopic mechanical properties. The finding that dislocation density reduction is the fundamental cause of softening is consistent with classical metallurgical theory but is here specifically validated for this pipe type. The practical implication is that annealing parameters (temperature, time, and cooling rate) should be optimized to maximize dislocation recovery while avoiding excessive grain growth that could compromise other properties.