1. General Introduction

For 600 MW and 1000 MW power plant electric-unit, the P92 alloy steel is widely applied in super-critical main steam line or reheating steam line. It can be furnished in accordance with ASTM A335 P92, ASME SA335 P92 or its EN equivalent X10CrWMoVNb9-2. Many piping structures of the high-temperature steam line require 3D mid-frequency hot-induction pipe bends made of ASTM A335 P92 materials. The bending quality, microstructure, as well as the reliability of the finished pipe bends mainly depend on the manufacture process control. Metals-Piping has supplied 4 pieces of R=3D ASTM A335 P92 bend to a power plant project in South China. All the four pipe bends are manufactured by mid-frequency hot-induction bending machine. Although all the fittings are tested okay, due to some fine adjustment of the manufacturing processes & conditions, the finished P92 pipe bends have distinctive difference in mechanical properties.

2. The Specification of ASTM A335 P92 Pipe Bends Required

Item No.	I.D x W.T	Center-Line-Radius	Angle	CLR/O.D Ratio
B1	248x53	1100	45°	3.1
B2	248x53	1100	45°	3.1
B3	254x62	1200	55°	3.2
B4	254x53	1200	62°	3.3

*All values are in mm unit unless otherwise specified.

3. The Bending Process of P92 Pipe Bends

Generally, the hot-forming temperature for ASTM A335 P92 or ASTM A213 T92 should be 850~1100°C [1560~2010°F]. Its forging temperature should be 950~1100°C [1740~2010°C] while the bending or extruding process should be conducted at 850°C~950°C [1560~1740°C].

Item No.	Heating-Temperature, °C	Bending Rate, mm/min	Cooling Method
B1	900	10~15	Air Cooling
B2	940	15~20	Air Cooling
B3	920	10~15	Air Cooling
B4	980~1040	10~15	Air Cooling

*The allowable heating-temperature variation for B3 can be ±10°C in practice.

4. Heat Treatment after Hot-Bending

Item No.	Normalizing	Tempering
B1	1050~1070°C, holding for 90 minutes.	750~770°C, holding for 150 minutes.
B2	1050~1070°C, holding for 90 minutes.	750~770°C, holding for 150 minutes.
B3	1050~1070°C, holding for 90 minutes.	750~770°C, holding for 150 minutes.
B4	1050~1070°C, holding for 90 minutes.	750~770°C, holding for 150 minutes.

*All the P92 pipe bends take the same heat treatment after its bending process(N+T).

5. Actual Composition, Hardness & Microstructure of Raw Pipes

Before the bending process, PMI has been conducted to determine the actual chemical composition & microstructure of the raw materials: 4 pieces of P92 seamless pipe. According to ASTM A335(ASME SA335), the maximum hardness of grade P92 is 250 HB. The hardness of the raw pipes are respectively 201 HB for B1, 201 HB for B2, 200~206 HB for B3, 180~185 HB for B4. The typical microstructure is tempered martensite with small amount of δ-Fe(≤5%). Besides the chemical composition are listed below:

Element	B1	B2	B3	B4
C	0.11	0.11	0.10	0.09
Si	0.20	0.20	0.20	0.29
Mn	0.43	0.43	0.47	0.44
P	0.007	0.007	0.013	0.018
S	0.002	0.002	0.005	0.008
Cr	8.56	8.56	8.88	8.61
W	1.52	1.52	1.70	1.84
Mo	0.37	0.37	0.48	0.41
Ni	0.39	0.39	0.19	0.30
Nb	0.06	0.06	0.075	0.06
V	0.15	0.15	0.21	0.23
Al	0.02	0.02	0.003	0.02
N	0.05	0.05	0.039	0.040
B	0.003	0.003	0.0021	0.0013

*All values are actually tested. Click to see the standard chemical composition requirements of ASTM A335 P92.

7. Inspection and Test for Finished Products

Inspecting Item		STD*	B1	B2	B3	B4
Roundness, %		≤5.0	/	/	≤1.1	≤0.8
W.T Reduction Rate, %		/	≤10.7	≤10.9	≤9.1	≤8.3
Corrugation Rate, %		≤2.0	0	0	0.6	0
Hardness, HB	E.A*	180 HB ~ 250 HB	210~229	215~234	187~202	213~220
	I.A*		189~218	221~234	196~202	213~220
	M.L*		192~242	215~243	181~228	210~218
MT*		Class I	OK	OK	OK	OK

*The above table shows the test results for the four finished A335 P92 pipe bends.
*MT:Magnetic Particles Test; *E.A/ I.A: the hardness value at the external/internal arc of the bend;
*M.L: the hardness value at the mid-layer of the bend; *STD: the standard value specified by relative Code.

You can also click to see the analysis on mechanical properties and optimum working conditions for the P92 steel in next article.