heat treatment process on surface hardness of 12CrlMoVG thick-walled seamless steel pipe

The relationship between microstructure distribution and hardness of 12Cr1MoVG thick-walled seamless steel pipe was analyzed; by measuring its CCT curve , the optimal heat treatment process for ① 219mmx45mm 12Cr1MoVG thick-walled seamless steel pipe was determined : quenching for 970 min and water quenching 20 s , tempering 750 oC"50min . The production results show that: by controlling the quenching time, the precipitation of ferrite can be reduced, and a large amount of martensite precipitation can be avoided, so as to control the hardness of the product; the new heat treatment process realizes 12Cr1MoVG The hardness of the thick-walled seamless steel pipe is stably controlled within 161 ~ 182 HB , the pass rate of the surface hardness reaches 100% , and its various mechanical performance indicators all meet the standard requirements.
Key words: seamless steel pipe; 12Cr1MoVG ; thick-walled steel pipe; metallographic structure; surface hardness; heat treatment process; quenching; tempering

12Cr1MoVG seamless steel pipe is mainly used to manufacture high-pressure boiler steam pipes, the service temperature can reach 580 C , and it has high temperature-resistant and durable strength Z] . 12CrlMoVG steel pipe is to add appropriate amount of alloy elements to high-quality carbon structural steel to improve its mechanical properties, strength and toughness.
Xiao Gongye ( 1964- ), male, professor-level senior engineer, has been engaged in the development, production technology and quality research of high-pressure boiler tubes, petroleum cracking tubes and nuclear power tubes for a long time.
Hardenability and hardenability, usually delivered in the state of heat treatment (normalizing + tempering or quenching + tempering) [ 5-9 ] . With the continuous improvement of user needs, it is required to
The surface hardness of 12CrlMoVG steel pipe is 145 ~ 190 HB , but the thick-walled steel pipe produced by traditional technology cannot meet this requirement (the hardness of steel pipe produced by normalizing + tempering process is 135 ~ 161 HB , which is lower than the lower limit of the standard; quenching + The hardness of the steel pipe produced by the tempering process is 175 ~ 200 HB , which exceeds the upper limit of the standard : 10 " 12 ] o
In order to obtain the ideal surface hardness of 12Cr1MoVG thick-walled steel pipe, by analyzing the continuous cooling transformation curve ( CCT curve) of 12Cr1MoVG , heat treatment process research was carried out in the laboratory, and the performance of steel pipes after heat treatment by different processes was tested to find out the best heat treatment process.

1. Situation Analysis
   1. Product organization distribution research

The hardness of steel is mainly determined by its metallographic structure, tempered sorbite
The S hardness is higher, and the ferrite F hardness is lower [ 13 ] . The metallographic structure of the 12Cr1MoVG thick-walled seamless steel pipe after traditional heat treatment is shown in Figure 1 .

from Figure 1 that the structure of 12Cr1MoVG thick-walled seamless steel pipe after quenching + tempering is S -back + B -back (tempered bainite, a large amount of M (martensite structure, after tempering After normalizing + tempering , the structure is F +P (ferrite + pearlite), and the F content is as high as 75% . Due to the fast cooling rate of the quenching process, F enters the B transformation area before it can be precipitated . Finally, the M+B structure is formed , and the hardness value of the product is relatively high; while the cooling rate of the normalizing + tempering process is slow, a large amount of F is precipitated, and the product hardness value is relatively low.

1. 1. CCT Curve Determination

The organization of the product is directly related to the cooling rate. In order to find the best heat treatment process, determine the CCT curve of 12CrlMoVG steel , as shown in Figure 2 . It can be seen that the product structure after quenching + tempering is mainly M , while the product structure after normalizing + tempering is mainly F+P , which is consistent with the metallographic structure shown in Figure 1 . Therefore, the cooling rate of the normalizing + tempering process should be appropriately increased , the residence time of the F transition temperature should be reduced, and the large-scale formation of F structures should be avoided, so as to achieve the goal of product hardness control.

1. experiment method

Specimen specification ① 219 mm x 45 mm , and its production process: electric furnace smelting T LF refining - VD furnace refining T die casting T forging T annealing - tube billet heating T piercing - Assel tube mill rolling T sizing T flaw detection. The 12CrlMoVG steel pipe that passed the flaw detection was selected , and a pipe section with a length of 500 mm was cut in the middle , and a sample meeting the corresponding standard was prepared after heat treatment in the laboratory. The chemical composition of the 12CrlMoVG sample is shown in Table 1 .

Table 1 Chemical composition of 12Cr1MoVG samples (mass fraction) %

Project C	Si	mn	P	S	Cr	Ni	Cu	Mo	A ( lt	V
Measured value 0.12	0.26	0.54	0.014	0.002	1.12	0.04	0.06	0.28	0.009	0.22
GB 5310-2008 Standard ⑻ 0.08 〜 0.15	0.17 to 0.37	0.40 to 0.70	W 0.025	W0.010	0.90 to 1.20	W 0.30	W 0.20	0.25 to 0.35	W 0.200	0.15 to 0.30

The experimental research is carried out in two steps: ① By controlling the residence time of F precipitation temperature and changing the precipitation amount of F , the influence of water quenching time and cooling water circulation mode on the performance of steel pipes is studied; ②Find out the water quenching time and The water circulation method is used to conduct research on the best tempering process system.

• Experimental Research

1. Determination of water quenching time
   1. Test purposes

The samples are heat treated in the laboratory, and water quenching is performed immediately after the samples are released from the furnace, and the water quenching time and water circulation mode are controlled. The effects of water quenching time and water circulation mode on the final structure of 12Cr1MoVG samples were studied, and the optimal quenching process for 12Cr1MoVG thick-walled steel pipes was determined.

1. Test Principle

According to the CCT curve, confirm that the quenching temperature of the sample is 970 ° C, keep it warm for 60 min , and perform water quenching immediately after it comes out of the furnace. Control the water quenching time to reduce the residence time in the F transformation temperature range, avoid a large amount of precipitation of F structure or completely generate M structure, so as to obtain the structure distribution that meets the hardness requirement. At the same time, the influence of the cooling water circulation mode on the product organization is compared and analyzed.

1. test results

The outlet water temperature and surface hardness of 12Crl MoVG thick-walled steel pipes were tested after different water quenching times and water circulation cooling , and the results are shown in Table 2 .
the steel pipe's water outlet temperature was 254 C after water quenching for 90 s , and its hardness value also met the requirements after testing. Metallographic structure of 12Cr1MoVG thick-walled steel pipe after water quenching at different times .
Table 2 Water outlet temperature and surface hardness of 12Cr1MoVG thick-walled steel pipes after different water quenching times and water circulation cooling

project	Water quenching time /s	state of water	Outlet water temperature / Wu surface hardness value HB
	60	Recycled water	203	189
	45	Recycled water	280	186
detection value	20	Recycled water	530	183
	90	non-circulating water	254	185
	45	non-circulating water	350	181
user request	-	-	-	145 - 190

1. 1. 1. Result analysis

from Table 2 that: the water quenching time is shortened, and the outlet water temperature of the steel pipe is increased. When the water quenching time is 20 s , the sample outlet water temperature is 530 °C, which can avoid a large amount of precipitation of the F structure; and because the quenching time is controlled, the F structure does not All of them are converted to M structure, which avoids the excessive surface hardness of the steel pipe and exceeds the requirements. Therefore, controlling the water quenching time at 20 s can achieve the best hardness value of 12Cr1MoVG thick-walled steel pipe. At the same time, compare the hardness of the product after water quenching with or without circulating water. Due to the relatively weak cooling capacity of non-circulating water, a large amount of m

from figure 3 that the shorter the water quenching time, the higher the F content, and the structure of the product after water quenching for 20 s is F+B , and the content ratios of F and B are 55% and 45% respectively ; due to the non-circulating water cooling capacity Relatively weak, after tempering, it avoids the formation of a large number of S -back structures, the F content is high, and the hardness is 181 HBo . The final quenching process is 970
Seven quenching, heat preservation for 60 min , non-circulating water quenching for 20 so
3.2 Determination of tempering process

1. Test purposes

The samples quenched by the above processes are tempered by different processes, and the best tempering process is found out through comparative analysis.

1. Test Principle

The sample was quenched at 970 Tx for 60 minutes , and immediately water quenched after being released from the furnace. The water quenching time was 20 s , and the tempering holding time was controlled. The performance and hardness of the sample after different tempering times were compared and analyzed to find the best tempering temperature that meets the hardness requirements. fire craft.

1. test results

The hardness test results of 12Cr1MoVG thick-walled steel pipes after heat treatment in different tempering process systems are shown in Table 3o
Table 3 Hardness testing results of 12Cr1MoVG thick-walled steel pipes after heat treatment in different tempering process systems

project	Tempering temperature / T	Holding time /min	Surface hardness HB
	750	90	185
detection value	750	120	185
	750	150	179
	750	180	177
user request	-	-	145 ~ 190

1. Result analysis

from Table 3 that with the extension of tempering holding time, the surface hardness of 12Cr1MoVG thick-walled steel pipe decreases gradually, from 185 HB to 177 HB , and the product hardness meets the requirements; the tempering time increases from 120 min to 150 min During the process, the hardness value decreased obviously, and the hardness decreased slightly when the tempering time continued to increase. Considering the comprehensive production cost and efficiency factors, the best tempering process is determined to be 750 Tx 150 mino
12Cr1MoVG thick-walled steel pipe produced by the above quenching + tempering new process , the hardness of the inner, middle and outer sections is tested. The test results are shown in Table 4. From Table 4 , it can be seen that the hardness values of the inner, middle and outer sections of the pipe body wall thickness are relatively uniform , and meet the product hardness requirements.
12Cr1MoVG thick-walled steel pipes after heat treatment by the new process were tested, and the results are shown in Table 5. From Table 5 , it can be seen that: quenching 970 ^ x 60 min , water quenching 20 s , tempering 750
Table 4 12Cr1MoVG thick wall after quenching + tempering

	Steel pipe hardness test results		HB
Measuring position	The outer surface	Intermediate wall thickness	The inner surface
1	179	174	179
2	180	174	181
3	177	175	178

12Cr1MoVG thick-walled steel pipes produced by the heat treatment process of 150 min meet the requirements.
Table 5 Conventional performance test results of 12Cr1MoVG thick-walled steel pipe after quenching + tempering new process heat treatment

project	Grain	Yield strong	Tensile strength	organize	hard surface
	degree / level	degree /MPa	degree /MPa		Degree HB
detection value	8.0	500	606	F+B back	179
Required value	5.0 ~ 10.0	M255	470 ~ 640	F+B back or B back +F	145 ~ 190

1. Production Practice

According to the test results, a new heat treatment process is implemented when producing 12Cr1MoVG thick-walled steel pipes: quenching at 970 Tx for 60 min , water quenching for 20 s , and tempering at 750 Tx for 150 min . water quenching. The hardness distribution of 12Cr1MoVG thick-walled steel pipes produced by traditional technology and new technology is shown in Figure 4 .
from Figure 4 that when the traditional process is adopted, the hardness of the steel pipe produced by the quenching + tempering process exceeds the required upper limit, up to 197 HB , and the batch rate exceeding the upper limit is 27% ; the normalizing + tempering process produces The hardness of the steel pipes is lower than the required lower limit value, the minimum is only 137 HB , and the batch rate lower than the lower limit required value is 41 % . . The mechanical properties and structure of the 12Cr1MoVG thick-walled steel pipe after heat treatment using the new process are tested , the results are shown in Table 6 , and all indicators meet the standard requirements.
Analyze the influence of product wall thickness on the water quenching process. Prolong the water quenching time when the wall thickness is greater than 45 mm , and shorten the water quenching time when the wall thickness is 36 ~ 45 mm , but not less than 18 s , otherwise the water quenching uniformity Difficult to guarantee.
5 Conclusion
( 1 ) By controlling the quenching time, reducing the amount of F precipitation, avoiding a large amount of M structure precipitation, and realizing the control of product hardness, it is determined that the best heat treatment process for ① 219 mm x 45 mm 12Cr1MoVG thick-walled seamless steel pipe is: quenching 970 Tx 60 min , water quenching for 20 s ,

( b ) new process
Figure 4 12Cr1MoVG produced by traditional process and new process
Hardness distribution of thick-walled steel pipes
Table 6 Mechanical properties and microstructure test results of 12Cr1MoVG thick-walled steel pipe after heat treatment by new process

project	Grain	Yield strong	Tensile strength	organize
	degree / level	degree /MPa	degree /MPa
detection value	7.0 ~ 8.5	395 ~ 525	550 ~ 610	F+B back
Required value	5.0 ~ 10.0	Three 255	470 ~ 640	F+B back or B back +F

Temper at 750 °C x 150 min . The product after using this heat treatment process
The tissue is F+B , and the proportions of F and B tissue are 55% and 45% respectively .
( 2 ) The new heat treatment process realizes the effective control of the hardness of 12CrlMoVG thick-walled steel pipe. After mass production verification, the product hardness value is 161 ~ 182 HB , the pass rate of surface hardness is 100% , and all mechanical performance indicators meet the standard requirements.

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