Technique Support

Contact Us

SINO ENGINEERING  GROUP(HK) CO., LIMITED
Beijing Add:18th  Floor ,Bright China Chang An Buliding  
158 Indoor Street,Jianguo Gate,Dongcheng  District ,Beijing China
Tel: +86 10-57196788Fax:  +86 10-57199088
Mobile:+86 185 14056500 +86 185 01918313
E-MAIL:sinoengineergroup@163.com
 
Skype:LCPIPES/sinoengineergroup
Website: www.sinoengineergroup.com 

Current Postion: Home > Technique Support > Products Specification
Products Specification

Method for improving property of weld of austenitic stainless steel

 

Method for improving property of weld of austenitic stainless steel

primary object of the present invention is to obtain a welded joint made of austenitic stainless steel that is superior in corrosion resistance property, by eliminating a chromium-depleted layer on one of the surfaces of the welded joint and changing the micro structure thereof to one containing the �� ferrite with an excellent corrosion resistance property, in consideration of the chromium-depleted layer and the high residual tensile stress generated in the heat-affected portion of the austenitic stainless steel. Also, this object is to provide a method for suppressing the generation of the SCC during the maintenance and repair of an austenitic stainless steel pipe of a plant that has been already provided.


   

In order to attain this object, according to a first feature of the invention, for a structure into which the corrosive fluid is to be brought into contact on both surfaces (e.g. inner and outer surfaces) of a heat-affected portion thereof after the austenitic stainless steel has been affected by the welding work, there is provided a method for applying a melting treatment to one surface of the heat-affected portion of the welded joint while cooling the other surface.


   

In accordance with a conventional welding method in which the pipe made of JIS SUS 304 (austenitic stainless steel) is cooled down to room temperature, a high residual welding tensile stress up to several times of 10 kg/mm2 is generated in an outer surface of the pipe by welding (refer to the welding portion 6) as shown by curve 8 in FIG. 2, in which TS denotes a tensile stress region and CS denotes a compression stress region. A line 9 denotes a reference level of 10 kg/mm. The chromium-depleted layer is formed in the heat-affected portion (or zone) 7 in the vicinity of the weld in a base metal 5a. Thus, if the corrosive fluid is brought into contact with both inner and outer surfaces of the base metal having the chromium-depleted layer and the high residual tensile stress, there is a large fear that the SCC would be generated in the heat-affected portion of the base metal.


   

Japanese Patent Examined Publication No. 59-21711 discloses one example of the method for coping with the SCC problem. In the method proposed in that publication, an corrosion resistant material containing delta (��) ferrite is overlaid by welding on surfaces of a plurality of stainless steel members to be weldingly bonded and brought into contact with the corrosive fluid in the vicinity of the weld. Subsequently, a melting treatment is performed onto the surfaces in contact with the corrosive fluid at toes of the overlaid layer with an input heat of 5 KJ/cm or less. Thereafter, the welding joint portion of the members made of stainless steel is welded. The purpose of this melting treatment is to eliminate the chromium-depleted layer to be generated in the heat-affected portion of the base metal by the overlay welding of the corrosion resistant material and to generate a formation that includes �� ferrite and is superior in corrosion resistance property. Also, a method set forth in Japanese Patent Unexamined Publication No. 53-56134 is similar to that disclosed in the above-described Publication No. 59-21711 in that the melting treatment is performed in the vicinity of the welding joint portion of the stainless steel prior to the welding work. Namely, the method is that the melting treatment is attained by imparting a heat energy such as arc and plasma to a surface layer to be affected by the welding work prior to the welding work of the members made of austenitic stainless steel. According to this method, the �� ferrite is generated when the molten portion through the melting treatment is solidified. Even if the micro structure obtained according to this method is affected by the welding heat, there is no sensitive region due to precipitation of carbides in the crystalline intergranular surfaces in the same manner as the application of the corrosion resistant material.


   

Another method for coping with the SCC is disclosed in Japanese Patent Examined Publication No. 60-45033. In that method, a corrosion resistant material containing 6 ferrite is overlaid by welding on surfaces of the welding members in contact with the corrosive fluid, and thereafter, a material is overlaid by welding on the surfaces opposite to toes of the previously overlaid layer (or bead), while cooling the surface of the latter layer side. The purpose of the overlay welding while cooling is to improve the residual stress in the chromium-depleted region generated in the heat-affected portion of the base metal by the first (or primary) application of the corrosion resistant material.


   

In accordance with a conventional welding method in which the pipe made of JIS SUS 304 (austenitic stainless steel) is cooled down to room temperature, a high residual welding tensile stress up to several times of 10 kg/mm2 is generated in an outer surface of the pipe by welding (refer to the welding portion 6) as shown by curve 8 in FIG. 2, in which TS denotes a tensile stress region and CS denotes a compression stress region. A line 9 denotes a reference level of 10 kg/mm. The chromium-depleted layer is formed in the heat-affected portion (or zone) 7 in the vicinity of the weld in a base metal 5a. Thus, if the corrosive fluid is brought into contact with both inner and outer surfaces of the base metal having the chromium-depleted layer and the high residual tensile stress, there is a large fear that the SCC would be generated in the heat-affected portion of the base metal.


   

According to this embodiment, it is possible to enhance the corrosion-resistance of both surfaces (e.g. inner and outer surfaces of pipe) of the stainless steel weld in the high temperature and high pressure water containing dissolved oxygen. Even if the high temperature and high pressure water containing the dissolved oxygen is brought into contact with the both surfaces of the stainless steel weld and the melting treatment (or remelting treatment) is possible only from one side of the joint, it is possible to insure the highly reliable stainless steel weld with a long service life.


   

In particular, the present invention is also effective for the case of maintenance and repair of the stainless steel pipe of the already built plant (the case where the high pressure and high temperature water containing dissolved oxygen is brought into contact with the both surfaces of the weld). The invention may be applied to various instruments or components of the nuclear reactor plant under the SCC generation environment.


   

As has been apparent from the foregoing description, according to the present invention, it is possible to enhance the corrosion resistance property on both surfaces (e.g. inner and outer surfaces) of the stainless steel weld. It is possible to obtain a long service life and highly reliable stainless steel weld even if the high temperature and high pressure water is brought into contact with the both surfaces of the stainless steel weld and the melting treatment (or remelting treatment) is possible only from one side thereof.