Development of P91 Hot-Rolled Seamless Steel Pipe

Literature Overview

This 2008 paper by He Biao, Xiao Gongye, Wang Guoliang, and Wu Guangxu from the Technical Center of Tianjin Pipe Group Co., Ltd. documents the development and trial production of P91 hot-rolled seamless steel pipes. Published in Tianjin Metallurgy (Issue 5, pages 37-40), the work provides detailed information on the chemical composition design, manufacturing process, mechanical property evaluation, and welding performance assessment of P91 seamless pipes intended for ultra-supercritical and ultra-ultra-supercritical power plant applications.

Core Technical Content

P91 steel (ASTM A335 P91 / GB 5310 equivalent) is a 9% chromium, 1% molybdenum, martensitic stainless steel that has become the material of choice for high-temperature components in advanced power generation systems. Its excellent combination of high-temperature strength, creep resistance, and oxidation resistance makes it ideal for steam pipes, superheaters, and other components operating at temperatures above 550 degrees Celsius.

Chemical Composition Requirements

Element	P91 Specification	Typical Trial Product	Standard Reference
C	0.05-0.12	0.08	ASTM A335 / GB 5310
Cr	8.5-9.5	9.1	ASTM A335 / GB 5310
Mo	0.85-1.05	0.95	ASTM A335 / GB 5310
Ni	0.3-0.6	0.45	ASTM A335
V	0.18-0.22	0.20	ASTM A335
Nb	0.04-0.10	0.07	ASTM A335
Si	0.20-0.50	0.35	GB 5310
Mn	0.30-0.60	0.45	GB 5310

Mechanical Property Evaluation

Test Condition	P91 Requirement	Trial Product Result	Standard
Tensile strength (RT)	585-965 MPa	620 MPa	GB 5310
Yield strength (RT)	415 min MPa	445 MPa	GB 5310
Elongation (RT)	15% min	18%	GB 5310
Tensile strength (600 degrees C)	355 min MPa	380 MPa	ASTM A335
Tensile strength (650 degrees C)	275 min MPa	295 MPa	ASTM A335
Charpy impact (RT)	47 J min	62 J	GB 5310
Brittle transition temperature	-27 degrees C	-27 degrees C	Internal specification
Microstructure	Tempered sorbite	Tempered sorbite	Visual inspection

Manufacturing Process Development

The development of P91 seamless pipe involves several critical process steps, each requiring careful control to achieve the required properties:

Process Flow and Critical Control Points

Process Step	Key Parameter	Control Objective
Steelmaking	Composition control	Meet specification within narrow ranges
Billet production	Temperature and cooling rate	Avoid segregation and ensure homogeneity
Piercing	Piercing temperature	1200-1250 degrees C for proper deformability
Rolling	Reduction ratio and speed	Achieve required wall thickness and uniformity
Heat treatment	Normalizing + tempering	Develop tempered sorbite microstructure
Sizing	Cold reduction	Achieve final dimensions and surface finish

The heat treatment cycle is particularly critical for P91 steel. Normalizing is performed at 1050-1100 degrees Celsius followed by air cooling to establish a fine martensitic structure, and tempering is conducted at 730-760 degrees Celsius to develop the tempered sorbite microstructure with optimal toughness and creep resistance. The brittle transition temperature of -27 degrees Celsius achieved in this development program indicates good low-temperature toughness, which is essential for safe operation under transient conditions.

Welding Performance Assessment

P91 steel is known for challenging weldability due to its high hardenability and susceptibility to cracking in the heat-affected zone (HAZ). The welding evaluation conducted in this study confirmed good welding performance, which is a critical requirement for field installation of power plant components.

Welding Process Requirements

Welding Parameter	Recommended Value	Rationale
Preheat temperature	250-300 degrees C	Reduce cooling rate, prevent martensite formation
Interpass temperature	250-300 degrees C	Maintain consistent thermal cycle
Post-weld heat treatment	760 degrees C, 2-4 hours	Temper HAZ, relieve residual stresses
Cooling rate (800-500 degrees C)	Less than 10 degrees C/s	Prevent hard martensite in HAZ
Fill metal	ER91S-B1 or equivalent	Match base metal composition

The requirement for post-weld heat treatment (PWHT) is non-negotiable for P91 weldments. Without PWHT, the HAZ may contain untempered martensite with hardness exceeding 400 HV, which is susceptible to hydrogen-induced cracking and has inadequate toughness. The PWHT cycle transforms the HAZ microstructure to tempered sorbite, reducing hardness to acceptable levels (typically below 300 HV) and restoring ductility.

Study Insights and Outlook

The successful development of P91 hot-rolled seamless pipe by Tianjin Pipe Group represents a significant achievement in China's capability to produce advanced materials for power generation. The trial product meets both domestic (GB 5310) and international (ASTM A335) standards, demonstrating that Chinese manufacturers can produce competitive P91 pipe products. From a practical standpoint, the key success factors identified in this development program include precise chemical composition control (particularly the Cr, Mo, and V content that governs creep resistance), appropriate heat treatment parameters to achieve tempered sorbite microstructure, and careful attention to welding process development to ensure field-installable weldments. The brittle transition temperature of -27 degrees C provides adequate margin for service conditions, and the good high-temperature tensile properties confirm the material's suitability for ultra-supercritical applications. This work provides a valuable reference for other manufacturers embarking on P91 pipe development programs, highlighting the importance of integrated process development that addresses composition, processing, heat treatment, and welding as a cohesive system rather than isolated tasks.