Cause of cracking of 310S stainless steel pipe

The content of chromium and nickel in 310S stainless steel is higher, and its corrosion resistance is better than that of ordinary 18-8 stainless steel. In boiling nitric acid with a concentration above 68.4%, the corrosion resistance of ordinary 18-8 stainless steel is not satisfactory, but 310S stainless steel can be applied in 65 ~ 85% nitric acid. 310S stainless steel is widely used in the nitric acid production equipment, such as the tail gas preheater, the heater, the condensing pipe, and the nitric acid producing equipment for the production of uranium oxide.

In addition, the 310S stainless steel has good thermal strength, oxidation resistance and resistance to vulcanization. The maximum application temperature is 1200 C, the continuous application temperature is even 1150 degrees C, and its heat resistance is much better than that of 304, 316, and 321 stainless steel. Compared to nickel chromium superalloy and cobalt chromium superalloy, its cost advantage is very obvious. But some aspects in the production are difficult to control, it is a poor heat conductivity and thermal conductivity of only 304 stainless steel 58%, 17 Cr 47%, crack of continuous casting billet tendency; the second is serious dendrite segregation, the maximum temperature of hot rolling heating will have some limitations, high temperature deformation resistance, thermoplastic low. In the rolling process is easy to form crack; third is chromium carbide precipitates, sigma tendency.

In the actual production, is an example of this is the use of electric furnace, LF furnace refining, continuous casting, hot rolling diameter is 90mm in diameter 310S stainless steel rod wall cracks in the heat pipe wear process, so do the analysis take inner wall cracking of 310S stainless steel pipe, so that the main reason for the cracking and some improvement measures.

The cracks in the head of the stainless steel pipe were observed, and a certain length of the pipe was extended from the head to the pipe to check the cracking of the inner wall. The samples were taken on the pipe and the chemical components were detected on the SPECTROLAB M10 photoelectric direct reading spectrometer after grinding. The metallographic specimen was tested at the crack, mainly the grade of inclusion, the observation of the crack micromorphology and the analysis of the deformation of the tissue. Samples with foreign bodies in the crack are used to analyze the chemical composition of the material with energy spectrum under scanning electron microscope. The preparation, grinding, etching, microscopic examination and microphotography of the sample are carried out according to the GB/T13298-1991 "metal microscopic examination method".

The data show that the high content of ferrite in stainless steel, low heat deformation temperature of pipe and fatigue use of molybdenum head will affect the cracking of 310S stainless steel pipe. We must use some measures to improve, first is the blank of the existing pipe before to try to increase the heating temperature, holding time, speed up the wear speed, need to check the molybdenum before production use; the second is to optimize the appropriate chemical composition, without increase of nickel content, lower chromium according to the standard control elements, residues of molybdenum elements can't be too much; third is because the alloy elements of 310S stainless steel with high content, serious dendrite segregation, coarse columnar crystals, low melting material and impurities easily intergranular and concentrated in the heart of the slab, in the smelting, casting molten steel cleaning after upgrading clarity in the premise of not completely exclude impurities or slag effect, reducing pouring temperature, superheat control, electromagnetic stirring technology, reduce dendrite segregation, improve the proportion of equiaxed grains It reduces the tendency of embrittlement caused by the segregation of impurity elements, improves the quality of the casting billet, and provides excellent quality for the subsequent heat processing.

Source: China Stainless Steel Pipe Manufacturer - Yaang Pipe Industry Co., Limited (www.yaang.com)