Heat Treatment Effects on Surface Hardness of 12Cr1MoVG Thick-Walled Seamless Steel Pipes

Literature Overview

This technical paper, published in the journal Steel Pipe (2015, Vol. 44, No. 3), presents a systematic investigation of heat treatment parameters for 12Cr1MoVG thick-walled seamless steel pipes manufactured by Tianjin Pipe Group Corporation. The research, led by Xiao Gongye and colleagues, addresses the critical challenge of achieving uniform and controllable surface hardness in thick-walled alloy seamless pipes, specifically for the specification Φ219mm × 45mm.

Core Technical Findings

The study analyzed the relationship between microstructure distribution and hardness in 12Cr1MoVG thick-walled seamless pipes. Through continuous cooling transformation (CCT) diagram determination, the optimal heat treatment parameters were established:

Parameter	Value	Purpose
Quenching temperature	970°C	Achieve full austenitization
Quenching time	60 min	Ensure thermal equilibrium through wall thickness
Quenching medium	Water	Rapid cooling for martensite formation
Quenching time in medium	20 s	Control martensite fraction
Tempering temperature	750°C	Relieve residual stresses, improve toughness
Tempering time	150 min	Achieve stable microstructure
Target hardness	161–182 HB	Within standard requirements
Surface hardness pass rate	100%	Consistent quality

The critical finding is that controlling the quenching time (60 minutes at 970°C) reduces ferrite precipitation while avoiding excessive martensite formation. This balance is essential for achieving the target hardness range while maintaining adequate toughness and ductility.

Technical Interpretation of Microstructure-Hardness Relationship

12Cr1MoVG is a Cr-Mo-V alloy steel designed for high-temperature pressure vessel and pipeline applications. The microstructure evolution during heat treatment follows a predictable sequence:

Austenitization phase: At 970°C, the steel transforms to austenite. The holding time of 60 minutes is critical for thick-walled pipes (45mm wall thickness) to ensure the entire cross-section reaches the target temperature and achieves uniform austenite grain size.
Quenching phase: Water quenching for 20 seconds produces a mixed microstructure of martensite, bainite, and retained austenite. The short quenching time in water limits the depth of martensite formation, preventing excessive hardness gradients through the wall thickness.
Tempering phase: At 750°C for 150 minutes, the martensite transforms to tempered martensite (sorbite), carbides precipitate and coarsen, and residual stresses are relieved. The high tempering temperature is appropriate for the target hardness range and ensures adequate creep resistance for high-temperature service.

Key Process Control Points

Control Parameter	Effect on Microstructure	Effect on Hardness
Quenching temperature	Grain size, austenite stability	Indirect through grain size
Quenching time	Ferrite precipitation, thermal uniformity	Reduced hardness with longer time
Quenching medium	Cooling rate, martensite fraction	Higher hardness with faster cooling
Tempering temperature	Carbide precipitation, tempering degree	Lower hardness with higher temperature
Tempering time	Carbide coarsening, stress relief	Slight hardness reduction

Engineering Practice and Quality Control

The achievement of 100% surface hardness pass rate demonstrates the effectiveness of the optimized process. However, several quality control considerations remain critical:

Thermal gradient management: For 45mm wall thickness, the thermal gradient during quenching creates differential microstructures between the outer surface and inner surface. The CCT diagram approach allows prediction of these gradients and selection of parameters that keep the entire cross-section within acceptable hardness limits.
Quenching distortion: Water quenching of thick-walled pipes can cause significant distortion and residual stress. The 20-second quenching time represents a balance between achieving sufficient cooling rate for hardness and limiting distortion.
Tempering uniformity: The 150-minute tempering time at 750°C ensures complete transformation of all martensite to tempered martensite. Insufficient tempering time can leave untempered martensite, which is brittle and susceptible to cracking.
Mechanical property verification: Beyond hardness, tensile strength, yield strength, elongation, and impact toughness must be verified to ensure the pipe meets API 5CT or ASME standards for high-temperature service.

Study Insights and Reflections

This study demonstrates the systematic approach required for heat treatment optimization of thick-walled alloy seamless pipes. The CCT diagram methodology provides a scientific foundation for process parameter selection, replacing empirical trial-and-error approaches. The emphasis on quenching time control as the primary lever for hardness management is a valuable insight for production engineers. In practice, the thermal mass of a 45mm wall thickness creates significant challenges for achieving uniform microstructure, and the 60-minute austenitization time reflects the need for thorough thermal penetration. Engineers working with thick-walled alloy pipes should always verify hardness profiles through the entire wall thickness, not just at the surface, as hardness gradients can indicate microstructure variations that affect mechanical performance and long-term service reliability.