34CrMo4 Seamless Steel Tube for Gas Cylinder

34CrMo4 gas cylinder has "bubbling" defects, and its chemical composition, defect formation mechanism, and production process are analyzed. The results showed that the defect was caused by the local heating at the end of the billet by the production plant . Through the improvement measures , the occurrence of such defects was effectively eliminated, and the yield of seamless steel pipes for 34CrMo4 gas cylinders was improved and stabilized .

Keywords : 34CrMo4 seamless steel pipe defect analysis

our company's 340 continuous rolling tube unit have the advantages of smooth inner and outer walls ,uniform wall thickness ,light weight ,high yield and low cost, and are very popular among users . According to market demand , medium carbon Cr and M have been developed successively . A series of seamless steel pipes for high-pressure gas cylinders, of which 34CrMo4 steel is a high-end product in GB/T18248 - 2008 seamless steel pipes for gas cylinders . The specification of 325 mm X7 mm is the limit specification of the 340 continuous rolling mill . The rolling is very difficult . If there is a little carelessness , the product will easily have some defects , which will reduce the pass rate of the product and seriously affect the overall yield of the product .

340 continuous pipe rolling unit produces 325 mm X 7 mm 34CrMo4 gas cylinder pipes, 1091 steel pipes (more than 1000 tons) are produced, of which 274 steel pipes have "bubbling" and crushing at the end 15 ~ 2 meters , resulting in less than 75 % product yield . Through the comprehensive analysis of these "inner bubbling" steel pipes ,the cause of this defect was found out , and rectification measures were proposed , which effectively prevented the occurrence of " inner bubbling" defects ,and improved and stabilized the product yield.

1. 34CrMo4 Steel Type Gas Cylinders
2. Billet source and production process
3. 80 tons of converter smelting + LF + VD + ^ 350 mm round billet continuous casting .
4. Using 80 tons of electric furnace smelting + LF+VD+ ^ 350 mm round billet continuous casting .
5. Outsourced 350mm rolling billets from Panzhihua Iron and Steel Co. , Ltd.

steel pipe rolling process
Tube billet inspection + tail end centering + ring furnace heating + head end centering + skew rolling piercing + ^ 340 mm restraint mandrel continuous rolling mill rolling + high pressure water descaling + sizing machine sizing + cooling + straightening .

1. defect analysis

Macroscopic morphology analysis
The length of the defect sample pipe was cut to be about 1.5-2 meters . According to the naked eye , there are a large number of bubble defects distributed longitudinally along the inner wall of the steel pipe .

Chemical composition analysis
The chemical composition results comply with GB/T18248 -
were taken near the defect , and the chemical composition of the 34CrMO4 steel grade in 2008 was chemically synthesized by direct reading spectroscopy .
The chemical composition near the inner surface defect of 34CrMo4 steel ( % )

C	Si	mn	P	S	Cr	M	Ni	Cu
Standard value 0. 30 ~ 0. 37	C a 40	0 60 ~ 0 90	C 0 020	C 0 020	0 90 ~ 1 20	0 15 ~ 0 30	C 0 030	C 0 020
Detection value 035	0 23	0 72	0 012	0 009	110	0 24	0 04	0 03

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