Study on Microstructure and Properties of P91 Tube Blank for High Pressure Boiler Seamless Steel Tube

Abstract: This topic analyzes and studies the low magnification, inclusions, microstructure, thermal compression deformation behavior and normal temperature mechanical properties of P91 tube blank samples.
The results show that the P91 tube billet with pure steel and low inclusion level can be prepared by adopting the process of "electric furnace + refining outside the furnace + vacuum degassing smelting" ; 91 has good hardenability, and can be obtained under air-cooling conditions. Martensite structure; 91 after high-temperature normalizing and tempering heat treatment, the structure is a sub-grained tempered martensite structure with fine grains; under the same strain rate condition, the deformation resistance is sensitive to the deformation temperature, and the higher When the deformation temperature reaches above 1 150 t , dynamic recrystallization of the austenite structure will occur as the degree of deformation increases, thereby further refining the grains ; P91 tube billet The normal temperature mechanical properties of the sample can meet the requirements of GB5310 and ASMESA335 standards.
Key words: P91 billet microstructure thermal compression deformation

P91 steel is a martensitic heat-resistant steel for boilers with excellent comprehensive performance jointly developed by the oak ridge national laboratory and combustion engineering company in the 1970s. It is based on 9Cr1 - Mo by adding V , Nb , N , etc. The elements are alloyed to generate a large number of carbon and nitrogen compounds such as Nb (C,N ) and V ( C,N ) that effectively hinder the movement of dislocations at the grain boundaries and within the grains . The steel has high oxidation resistance, high temperature steam corrosion resistance, good impact toughness and long-lasting plasticity. Compared with austenitic stainless steel, it has a lower thermal expansion coefficient and higher thermal conductivity than austenitic stainless steel , which is mainly used for superheaters, reheaters, high-temperature pipes and headers for power plant boilers.

2011 , TISCO successfully developed a large-diameter P91 billet for high-pressure boiler seamless steel pipes, realizing a breakthrough in TISCO's production capacity from small-diameter T91 billets to large-diameter P91 billets. This topic mainly studies the microstructure and properties of P91 tube blank samples, in order to gain a deep understanding of the characteristics of the steel, and provide a basic guarantee for the subsequent tube-making process and performance improvement.
1 Experimental method
The grade of the tube billet trial-produced in this experiment is P91, and the corresponding Chinese grade is 10Cr9Mo1VNbN , and its chemical composition is shown in Table 1 . The alloy raw materials for tube billet are smelted in EBT electric furnace and refined outside LF furnace, and then die-cast into steel ingot after VD vacuum degassing treatment. The steel ingot is formed into a material after forging .

Table 1 Chemical composition of P 91 :

w (C)

w (Mn)

w (Si)

w (S)

w (P)

w (Cr)

w (Mo)

w (V)

w (Nb)

w (Al)

w (N)

w (Ni)

w (Cu)

0.08~0.12

0.30~0.60

0.20~0.50

< 0.010

< 0.020

8.00~9.50

0.85~1.05

0.18~0.25

0.06~0.10

< 0.04

0.03~0.07

< 0.40

< 0.25

Samples were taken from the prepared tube blanks, and the low magnification, inclusions, microstructure, thermal compression deformation behavior and mechanical properties at room temperature of the samples were analyzed and studied.
2 Analysis and research of experimental results

1. Low magnification and inclusions After low magnification inspection, the acid leaching low magnification tissue sample of the cross-section of the tube blank has no visible defects such as white spots, shrinkage cavity residues, delamination, cracks, bubbles, inclusions, skin turning and subcutaneous bubbles. The tube blank has a good low-magnification structure, and the general porosity, center porosity, and segregation are not more than 0.5 grades, as shown in Table 2 .

	Table 2 Low magnification analysis		class
Specification	General loose	loose center	Segregation
standard	< 2	< 2	< 2
Φ500mm _	0.5	0.5	0

Table 3 is the analysis of the inclusion level of the P91 tube blank sample. It can be seen from Table 3 that the grades of various inclusions in the tube blank are low, there are no inclusions of types A , B , and C , and the level of inclusions of type D is also reduced. This fully shows that the steel smelted this time is pure, and provides a guarantee for the good comprehensive performance of the subsequent P91 steel.
Table 3 Inclusion Levels

Specification	A		B		C		D.
	thick	thin	thick	thin	thick	thin	thick	thin
Φ500mm _	0	0	0	0	0	0	0	0.5

After flaw detection, the defect equivalent of the tube blank is less than 0.4 mm, and the defect equivalent is small. The tube blank has been fully forged and deformed, and the original shrinkage cavity and porosity of the steel ingot have been eliminated. The good internal quality of the tube blank also provides a guarantee for smooth tube making.

1. Microstructure

Figure 1-1 shows the microstructure of the P91 tube billet sample after being held at 1050 °C for 30 minutes and air-cooled. It can be seen from this figure that the microstructure of the steel after air - cooling is martensite , because the steel contains A large amount of Cr , Mo and other elements make the steel have good hardenability, and the martensite structure can be obtained at a low cooling rate, and the structure is fine, which shows that the tube blank has been sufficiently forged and deformed. Figure 1-2 shows the microstructure of the P91 tube sample after normalizing and tempering at high temperature . The martensite structure has high strength and toughness, and at the same time, there are high-density dislocation lines distributed on the structure, which further improves the strength.

1. Thermal Compression Deformation Behavior

Gleeble 3800 thermal simulation testing machine was used to study the relationship between deformation degree and deformation resistance under a certain strain rate ( 0.1 s -1 ) and different deformation temperatures. Under the condition of reducing the deformation temperature, as the degree of deformation increases, the deformation resistance increases; under the same strain conditions, the deformation resistance decreases with the increase of temperature, as shown in Figure 2-1 . Under the condition of higher deformation temperature above 1 150 °C , dynamic recrystallization of austenite structure will occur as the degree of deformation increases, and the deformation resistance increases with the increase of deformation degree. When the degree of deformation increases to a certain extent, The degree of work hardening of the austenite structure is offset by the degree of recovery softening, and the deformation resistance tends to a certain value. When the degree of deformation is further increased, the degree of dynamic recrystallization softening of the austenite structure is greater than the degree of work hardening, and the deformation resistance begins to decrease. As shown in Figure 2-2 .
It can be seen from the above that the deformation resistance decreases significantly with the increase of deformation temperature. Therefore, under the condition of no overheating, the deformation temperature should be as high as possible to reduce the deformation resistance during forging. Under this condition, dynamic recrystallization is easy to occur, thereby further refining the grain structure.

1. Normal temperature mechanical properties

on the P91 tube blank sample. In order to ensure that most of the carbides were fully dissolved in the austenite and at the same time avoid coarsening of the austenite grains, the normalizing temperature was selected as 1050 ° C. The time is 30 min ; the tempering temperature is selected as 770 °C , and the temperature is kept for 50 min , so that after tempering, the carbides in the matrix structure are uniformly precipitated, and the martensite structure is further subcrystallized, so as to improve the comprehensive mechanical properties of the material. After high temperature positive tempering heat treatment, the experimental results of longitudinal mechanical properties of the samples at room temperature are shown in Table 4 . The results show that the prepared tube has good mechanical properties at room temperature, and the mechanical properties at room temperature can meet the requirements of GB5310 and ASME SA335 standards.
Table 4 P91 sample mechanical properties (room temperature)

sample	R el /MPa	R m /MPa	A /%	A kv /J	H B
GB5310 , ASME SA335	> 415	> 585	> 20	> 40	< 250
P91 sample	655	805	twenty one	150	157

3 Conclusion
1 ) P91 steel billets with pure steel quality and low inclusion level can be prepared by adopting the process of "electric furnace + refining outside furnace + vacuum degassing smelting" .
2 ) P91 has good hardenability, and martensite structure can be obtained under air-cooling conditions ; P91 is sub-grained and tempered martensite structure after high-temperature normalizing and tempering heat treatment.
3 ) Under the same strain rate condition, the deformation resistance is more sensitive to the deformation temperature, and the higher deformation temperature can reduce the deformation resistance in the forging process; when the deformation temperature reaches above 1 150 °C , the Dynamic recrystallization of the austenite structure occurs, resulting in further grain refinement.
4 ) The normal temperature mechanical properties of the P91 tube blank sample can meet the requirements of GB5310 and ASME SA335 standards.
Table 2 Rolling schedule of H300 x 150 billet mill after improvement

project	1 pass E pass	2 passes B hole type	3 passes B hole type	4 passes B hole type	5 passes B hole type	6 passes A hole type	7 passes A hole type
Roll gap /mm	215	76	65	51	39	34	31
Linear speed / ( m・s -1 )	2.2	2.8	3.2	3	3.2	3.2	3.4
Centering position /mm	150	1 000	1 020	1 050	1 050	1 565	1 600
Rolled piece width /mm	400	456	460	468	477	482	488
Rolled piece height /mm	350	227	212	198	188	170	167
Rolling method	vertical rolling	flat rolling	flat rolling	flat rolling	flat rolling	flat rolling	flat rolling
Work roll diameter /mm	648	816	816	815	814	823	823

Rolling machine and universal rolling mill, the finished product size fully meets the national standard requirements.
From April 2010 to December 2010 , a total of
of H300 x 150 series products is about 45,000 tons , and the one-time rolling volume after roll turning of the blanking machine reaches 100,000,000 tons .

 epilogue
Aiming at the problems in the actual production of H300 x 150, through continuous analysis and improvement, the smooth flow of the production process is ensured, which not only trains engineers and technicians, improves the ability to deal with problems, but also provides reference for similar domestic production lines .

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