5.0.1 The allowable lowest ambient temperatures during welding are as follows:

   low carbon steels                            -20℃

   low alloy steels and common low-alloy steels    -10℃

   medium and high alloy steels                   0 ℃

5.0.2 The preheat  temperature of various steel before being welded are shown in Table 5.0.2 ; additionally the following stipulations shall be observed :

 Table 5.0.2     Preheat Temperature

5.0.2.1 The preheat requirements shall be raised based upon the results of welding technological test ;

5.0.2.2 For alloy steel pipes and their ancillary components （e.g. tee-piece , bent pipe）with wall thickness≥6mm to be welded under an ambient temperature below 0℃ the preheated temperatures shall be raised up 20~50℃ above the stipulated values given in Table 5.0.2 ;

5.0.2.3 Low alloy steel pipes with wall thickness <6mm and low carbon steel pipes with wall thickness>15mm to be welded under an ambient temperature below 0℃,shall also be appropriately preheated ;

5.0.2.4 While welding up different type steels , the preheat temperature shall be selected in accordance with that material with poorer weld ability or higher content of alloy ;

5.0.2.5 In welding up main pipe with stub , the preheated temperature shall conform with the stipulation for main pipe ;

5.0.2.6 In welding up unpressurized components onto the pressure vessel , the preheated temperature shall be selected according to the pressure vessel .

5.0.3 The width of preheat band shall be measured from the center of butting extending , not less than 3 times of weldpiece wall thickness on each side .

5.0.4 In the course of welding, the temperature of interpass shall not be lower than the lower limit of preheat temperature and not higher than 400 ℃ .

5.0.5 In order to assure the inner side of pipe cleaned and the root layer quality of seam , the welding method shall conform with the stipulation in Table 5.0.5 for pressure piping welding .

 Table 5.0.5      Welding Methods of Pressure Pipe

5.0.6 For medium or high alloy steel pipes (Cr content ≥ 3％ or total alloy content >5% ) , argon or mixed gas shall be fill in to prevent the root layer from being oxidized or overburnt .

5.0.7 It is strictly forbidden to initiate arc , test the electric current amount or weld temporary supports on the work pieces to be welded.

5.0.8 While tube/pipe being welded , there shall not be air-flow blowing through.

5.0.9 In tack-welding at root position of butt joint , similar requirements regarding welding materials , technology , operator and preheated temperatures as those stipulated for normal welding shall come into effect . In addition , the following requirements shall be met :

5.0.9.1 After tack-welding at root position of butt joint, the quality of each welding point shall be inspected . When defects being discovered , it shall be remove at once and tack-welding shall be done again .

5.0.9.2 Care shall be taken to prevent damaged to base metal while removing temporary tack used for tack-welding of large wall thickness pipes . The residual weld deposit shall be removed off and repaired .

5.0.10 While tungsten argon arc welding being employed for root layer seam , in order to avoid the occurrence of cracks , welding of next layer seam shall be timely carried out once the previous layer has been inspected .

  In welding up multi-layer and multi-run seams , attention shall be paid to the defects occurring in each layer , and proceeding to the next layer can be continued only after the seam quality being ascertained qualified by operator’s own inspection.

5.0.11 The pipe of large wall thickness and diameter shall be welded with multi-layer and multi-run welding , when the thickness of the pipe more than 35mm , the following requirement shall be met :

5.0.11.1 The thickness of root layer welded by tungsten argon arc welding shall be not less than 3mm;

5.0.11.2 The thickness of the rest layer of other pass shall not more than the electrode diameter plus 2mm;

5.0.11.3 The oscillation width of single pass shall not more than five times of diameter . For the arrangement of multi-run pass of large wall thickness pipe see the Fig. 5.0.11.

       Fig. 5.0.11  Pass arrangement for welding up thick-wall pipes

5.0.12 In order to minimize welding deformation and defects , it is preferable that welding by two operators simultaneously at symmetrical positions for butt welding of pipes with diameter larger than 194mm is to be resorted.

5.0.13 The welding of steel structure shall conform with welding procedure specifications .

5.0.14 In the course of welding , special attention shall be paid to the quality of joint and arc extinction , the joint of multi-layer shall be stagger , while extinguishing arc , the fused bath shall be filled up.

5.0.15 Welding shall be carried out continuously to completion except interruptions necessary for technology or inspection . In case of interruptions , measures （e.g. , post-heating , slow-cooling , or thermal insulation）shall be taken to prevent the occurrence of cracks . Close inspection on whether any crack occurs shall be carried out before welding proceeding .

5.0.16 For butt welding of pipes or vessels with nominal diameter>1000mm,double-side welding at symmetrical positions shall be employed . In order to ensure the quality of sealing end welding , root layer of welding seam shall be cleaned up , and the oxide shall be removed .

5.0.17 For the concealed welding seams which have to be inspected , inspection shall be done to verify their qualification before going on the next process.

5.0.18 On the completion of welding , a clear up shall be done around the seam which is to be inspected by the operators on their own for qualifications . The embossed stamp bearing the operators number or designation can then be applied near the welding seam.

5.0.19 For the steel which is liable to form delayed crack , heat treatment shall be applied soon after welding . Otherwise , post-heat-treatment shall be required with a temperature preferably to be 250-350℃,followed by a slow cooling process and thermal insulation . The scope of heating is the same as that of heat-treatment.

5.0.20 In case of defects exceeding the allowable range , mending by cutting out and patching can be resorted as remedial measures . However , such mending can not be applied more than three times （twice for medium and high alloy steel）at the same location , and shall be done in accordance to the stipulations as follows :

5.0.20.1 The defects are to be removed thoroughly;

5.0.20.2 Mending weld shall be carried out according to the technological requirements , and specified procedures of mending .

5.0.20.3 For welds requiring immediate heat treatment , heat treatment shall be done once again after mending .

5.0.21 The tools used for fixing cold drawn welding gap of pipes shall be dismounted only after completion of en bloc butt welding and heat treatment.

5.0.22 Do not allow to correct the welding joint by heating.