Analysis of cracking after martensitic stainless steel

In recent years, with the rapid development of China's electric power and boiler industry, the demand for boiler tube billet continues to increase.10Cr9Mo1VNb is martensite type stainless steel, with high oxidation resistance and high temperature steam corrosion resistance, good impact toughness and lasting plasticity, used as subcritical, supercritical boiler wall temperature 620℃ high temperature petroleum cracking device furnace pipe, superheater, reheater steel tube, high temperature container, steam pipe, nuclear power heat exchanger, etc. Our factory recent steel forging tempering steel burst phenomenon, sometimes even the whole through long burst phenomenon, according to the understanding of the causes of forging cracking and summary, now from the annealing furnace heating, different heat treatment system contrast control situation and steel microorganization and performance relationship between analysis and research.

1. Technical requirements

1.1 Chemical composition (Smelting analysis)

1.2 Manufacturing method

Using electric furnace smelting and furnace refining and vacuum degassing treatment, the ingot must be forged, the forging ratio is greater than 3.

 

2. Production process

After the hot steel ingot reaches the forging steel company, it will load the cooling pit and cover as soon as possible to avoid the ingot cooling too quickly. After the slow cooling time arrives, transfer to the standby furnace, the temperature is 650~750℃, 8h. The heating temperature is 1180~1200℃, with large pressure amount as far as possible. Forging and tempering is not allowed to extend the furnace loading time for some reason, the furnace loading time is 40min.

 

3. Sampling analysis of explosion causes

Four 10Cr9Mo1VNb pipe billet (furnace number is 160760,160786,161170, respectively) in the forging ignition, in the furnace number is 160786 sampling for analysis and identification.

3.1 10Cr9Mo1VNb tube blank burst low power test

The pipe blank is broken into smooth cracking and full length cracking. The crack depth has exceeded the radius, and no other metallurgical defects are found.

3.2 Metlographic analysis and test verification

The microtissue of the sample of the forging and the hardness test of 3 samples and a normal 10Cr9Mo1VNb sample without cracking. The result showed that the edge, half radius and core hardness values reached & gt; HB 380. The normal sample hardness value is HB 170.

Normal sample tissue: diffusion dehydrogenation 680℃ annealed tissue is ferrite and granular carbide; SA-335 / SA-335M standard called 10Cr9Mo1VNb ferrite alloy steel, the ignition state shall be

ferritic structure. The test showed that the 20mm×10mm sample was insulated at 1050℃ for 50min, and the air-cooled normal tissue was a slab of martensite.

The tissue is basically the same, the sample is 1050℃ normal, quenching, 780℃ air cooled tempering, martensite decomposition, but still directional.

According to the metallographic test results, the tissue of the burst test steel sample is martensite, while in the normal 10Cr9Mo1VNb steel annealing state, the tissue is not normal. Therefore, do the verification test: 1080℃ insulation 1.5h water cooling, air cooling, 1080℃ insulation 1.5h respectively to 650℃, 680℃, 710℃ insulation for 20h furnace cooling, tissue and hardness test, the results are shown in Table 2.

Organization observation and hardness test shows that water cooled, air cooled sample completely formed martenite, 650℃ insulation without ferrites + carbide transition, and the same state of air cold tissue, hardness is similar, 680℃ insulation part into iron, most or martensite tissue (see figure 1), 710℃ insulation basically into iron + carbide (see figure 2).

4. Cause analysis

Because 10Cr9Mo1VNb steel contains a large number of Cr, Mo, V, Nb and other alloy elements to make the bead body transition curve move right, the critical cooling speed is low, the austenite stability is great, and the MS point is improved. According to the continuous cooling curve of the 10Cr9Mo1VNb steel, the Ms point should be close to 40℃, and the lowest point of the temperature of the curve of the A to F + C transition is close to 680℃. It can be considered that the annealing tissue above 680℃ is ferrite + carbide, while the transition from A to F + C, which can only maintain the austenite state and form the martensite tissue in the subsequent cooling. The experiments we did also proved that 710℃ insulation for 20h was completely transformed into ferrite + carbide, while 680℃ insulation for 20h. Due to insufficient time, only partially converted into ferrite, most of the tissue was martensite, which A to F + C did not occur at all.

Our current heat treatment system is 600℃ material after forging, heating to 680℃ for 70h, and 30℃ / h to 150℃ pit. Normally microtissues should be ferroite + carbides. Hardness was from HB 180 to HB 200. The tissue of the burst sample is martensite, with the hardness of HB 385. Through the production site investigation, it was found that 4 steel pieces of burst were placed in the low temperature area of the furnace.

Through the above analysis that: due to the uneven temperature of annealing furnace, make the insulation temperature of individual steel is far lower than 680℃, although the insulation time is longer, but still cannot occur in the heat preservation stage of austenite to ferrite + carbide, can only keep the austenite state, in the subsequent cooling process although speed is slow, the temperature is very low but still happen of martensite. Therefore, there is a great residual stress after cooling. Due to the slow cooling speed, thermal stress should be well released, and the tissue stress dominates in the residual stress, that is, the heart is subjected to tangential compressive stress, the surface is subjected to tangential tensile stress, when the residual stress exceeds the tensile strength, the burst.

 

5. Conclusion

(1) The 10Cr9Mo1VNb tube billet is cracked because the realing temperature is low.

(2) The temperature of the heat treatment furnace is uneven, so that the pipe billet heated in the area far below the thermal insulation temperature forms martensite tissue when cooling, and there is a large residual stress burst.

6. Improvement measures

(1) Increase the forging annealing temperature, and use the annealing furnace with a more uniform furnace temperature.

(2) Raise the annealing temperature appropriately to compensate for the uneven furnace temperature.