Research on heat treatment process of 15NiCuMoNb5-6-4 seamless steel pipe

Abstract In this paper, the effect of heat treatment process on the properties of 15NiCuMoNb5-6-4 is studied . It is found that with the increase of tempering temperature, the hardness first decreases and then increases, and the strength gradually decreases; with the extension of tempering holding time, the hardness gradually decreases, and the impact toughness gradually increases; the results show that the best tempering process for this steel is tempering The temperature is 630 ~ 660 ° C , and the holding time is not less than 90 minutes . At this time, the steel has the best comprehensive performance.
Keyword 15NiCu
15NiCuMoNb5-6-4 belongs to the steel grade of European standard EN10216-2 . It adds Ni-Cu-Mo-Nb and other alloys on the basis of carbon-manganese steel , which has the effect of fine-grain strengthening and precipitation strengthening [1] , and greatly improves the high-temperature strength and durability of the steel. The steel is characterized by high strength and a service temperature of 400° C. It can also be used as a high-stress pipe with a pipe wall temperature of 500° C , or as a high-parameter thermal power generation unit. It is widely used in boiler steam drums in supercritical units, High pressure components such as steam separators, headers, steam generators, steam pipelines and nuclear power equipment.
In this paper, by analyzing the composition characteristics of 15NiCuMoNb5-6-4 , the heat treatment process was studied, and the optimal heat treatment process was obtained.

1. experiment method

The specific tube-making process is: electric furnace - LF - VD - die casting-forging-annealing-tube billet heating- piercing - PQF continuous rolling to determine the diameter-flaw detection, the specification is 0 323.9x14.2mm . The chemical composition is shown in Table 1 .
Table 1 Chemical composition of samples used in the test (wt/%)

element	C	Si	mn	P	S	Cr	Ni	Cu	Mo	Nb	Al tot
standard	< 0.17	0.25 ~ 0.50	0.80 ~ 1.20	< 0.025	< 0.020	< 0.30	1.00 ~ 1.30	0.50 ~ 0.80	0.25 ~ 0.50	0.015 ~	< 0.040
										0.045
Measured	0.14	0.43	0.96	0.005	0.003	0.26	1.14	0.62	0.32	0.020	0.011

The test materials are taken from steel pipes that pass the flaw detection after rolling, and are formulated in accordance with the relevant standards of the national standard.
The C curve was measured for 15NiCuMoNb5-6-4 steel , as shown in Figure 1 . It can be seen from Figure 1 that under normal normalizing conditions, the tissue is basically
B+F , some M may appear after quenching .

1. test results

According to the requirements of the European standard EN 10612 , combined with the C curve of the steel , the effects of tempering temperature and holding time on the properties and structure of the steel were studied.
By studying the C curve, this paper fixes the normalizing temperature at 920° C ( Ac3 is about 850° C ), and the holding time is 20min . The mechanical properties after different heat treatment processes are shown in Table 2 .
Table 2 Mechanical properties and microstructure after different heat treatment processes

heat treatment process	Yield strength Rp0.2MPa	Tensile strength /MPa	Elongation after break A	Impact energy Ak/J	hardness (HBW10)
EN10216-2	on 440	610~780	19	on 40
Temper at 600 C , hold for 2 hours	495	665	25.5	165	217
Temper at 630 C , hold for 2 hours	500	660	22.5	176	213
Temper at 660 C , hold for 2 hours	475	670	24.5	170	200
Temper at 680 C , hold for 2 hours	410	645	25.0	110	202
660 C , hold for 60 min	440	695	23.5	119	211
660 C , hold for 90 min	455	675	23.0	165	208

1. Result analysis
   1. Effect of Tempering Temperature and Holding Time on Hardness

The influence of different tempering temperature and holding time on hardness is shown in Figure 2 . When the temperature is not higher than 660 C , the hardness decreases obviously with the increase of tempering temperature. At this time, the grain recovery occurs, the dislocation density decreases, and the ability of the material to resist deformation decreases; but when the tempering temperature reaches 680 C , the hardness decreases. It also increased slightly, and no phase transformation occurred at this time ( a ci is about 725 C ), so this change may be caused by the precipitation of carbides. The hardness decreased significantly with the extension of the holding time.
The metallographic structure after normalizing and tempering is shown in Figure 3. With the extension of tempering temperature and holding time, the metallographic structure basically does not change, which is B+F .

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