Metal treatment – Stock – Ferrous
Reexamination Certificate
1998-10-19
2002-08-06
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C420S094000, C420S097000, C420S119000
Reexamination Certificate
active
06428633
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a steel for welded structures (steel for a welding base material), a welding wire, and a multipass welding process which decrease the residual stress of a weld zone to improve the reliability of welded structures, and which improve such various properties of welded structures, to which the residual stress is related, such as stress corrosion cracking characteristics, fatigue characteristics and brittle fracture characteristics.
2. Description of the Related Art
The most common method for decreasing the residual stress of a weld zone has been to conduct a post-weld heat treatment (PWHT) (=stress relief annealing (SR)) after finishing the preparation of a welded joint. The PWHT not only decreases the residual stress but also improves the metallurgical characteristics. However, since the PWHT also becomes a chief factor increasing the production cost of welded structures, it would be economically very advantageous to obtain predetermined characteristics of the welded structures without PWHT.
Methods for obtaining the predetermined characteristics without PWHT can be roughly classified into two categories. A method which belongs to one of the categories is one for improving various properties such as the resistance to stress corrosion cracking and the fatigue strength of a welding base material and/or welding material (such as a welding wire) so that the predetermined characteristics can be obtained even when the residual stress caused by welding is a tensile one, and many proposals therefor have heretofore been presented.
A method which belongs to the other category is one for improving a welding material (welding material) and/or welding process to decrease the residual stress caused by welding. The method utilizes the phenomenon that the stress corrosion cracking characteristics are improved when the residual stress is in a compressive state. Representative methods belonging to the category are as follows: a method which utilizes a transformation expansion produced when a weld metal transforms from austenite into martensite; a method such as a shot peening method which mechanically decreases the residual stress after welding; a method in which a weld bead is newly formed intentionally on a portion of a joint the characteristics of which cause problems, so that a compressive residual stress is distributed in the portion; and the like. Of the methods mentioned above, the latter two have the problem that in addition to the work of preparing welded structures, further work must be carried out.
On the other hand, investigations have been carried out into the method for improving welding materials and/or welding processes so that the residual stress is decreased or made a compressive one, as published, for example, in “Yosetsu Gakkai Zenkoku Taikai Koen Yokoshu (the Proceedings of the National Conference of the Japan Welding Society),” vol. 51 (1992) pages 278-279.
In the conventional process for decreasing the residual stress, the temperature (Ms temperature) at which transformation from austenite into martensite starts in a weld zone is lowered, and the transformation expansion is allowed to take place at low temperature, whereby the residual stress is decreased. The thermal contraction of the weld zone which is the cause of the residual stress is counterbalanced by an expansion involved in the martensite transformation. The thermal contraction is thus temporarily reversed into an expansion in the course of lowering the temperature, to decrease the residual stress.
However, even when the Ms temperature of a welding base material or welding material (such as a welding wire) is significantly low, it is difficult to substantially decrease the residual stress for the reasons described below. Even when the weld zone temporarily reaches a compressive stress state due to a martensite transformation expansion at the Ms temperature in the course of lowering the temperature, a thermal contraction is produced again in the course of lowering the temperature after completion of the transformation, and the weld metal reaches a high tensile stress state again.
As explained above, decreasing the residual stress with a steel material or welding material alone is restricted to materials having a very low Ms temperature in the prior art. When such materials are to be used, such materials must often be prepared by adding alloying elements to such a degree that the addition is not suitable for practical use. Therefore, designing a new material which does not rely on a low Ms temperature alone is desired, which is a background of the present invention in a first aspect.
Furthermore, when multipass welding is conducted, the residual stress of each of the weld beads in the final layer is not always surely decreased, and complete avoidance of the partial generation of a high residual stress is impossible even when a material having a significantly low Ms temperature is employed for the reasons described below. When a plurality of weld beads are present in the weld zone, they interact each other. As a result, even if the residual stress is decreased in some beads, the residual stress of the other beads is not decreased. Such a problem arises either in the case of using a material having a very low Ms temperature as practiced in the prior art, or in the case of using a material of new design. This is a background of the present invention in a second aspect.
As explained above, when such a method for decreasing the residual stress with a steel material or welding material alone is to be practiced, alloying elements must be added to an impractical extent. Moreover, even when such a steel material or welding material is used, there are still problems, for example, the transformation expansion of a weld zone cannot be effectively used for the residual stress due to the interaction among weld beads as in the case of multipass welding. As a result, the method for decreasing the residual stress in a weld zone has not been sufficiently established in practice.
However, even when those welding base materials and welding materials (such as welding wires) which have improved properties and which have been proposed are used, the additional use of decreasing the residual stress is extremely favorable for improving the reliability for the welded joints. Stress concentration is likely to take place in a weld zone to form a fracture initiation point. It is, therefore, extremely effective from the standpoint of improving the reliability of welded structures as a whole to decrease the residual stress of a weld zone which is present as a tensile stress on the surface of the welded structures.
SUMMARY OF THE INVENTION
An object of a first aspect of the present invention is to provide a steel for welded structures, and a welding wire, which can decrease the residual stress of a weld zone.
An object of a second aspect of the present invention is to provide a multipass welding process which solves the problem that, in a multipass weld zone, even when a material for decreasing the residual stress is used, the residual stress becomes partially high as a result of the interaction between weld beads.
According to a first standpoint of the first aspect of the present invention, the following steel is provided.
[1] A steel for welded structures which starts a transformation from austenite into martensite at a temperature from at least 200° C. to up to 350° C., and which has a yield strength from at least 60 kg/mm
2
to up to 120 kg/mm
2
at the transformation starting temperature.
[2] The steel for welded structures as stated in [1], wherein a parameter Pa defined by the formula
Pa
=C+Ni/12+Cr/24+Mo/19
wherein C, Ni, Cr and Mo represent respective element contents in terms of percent by weight, is from at least 0.85 to up to 1.15.
[3] The steel for welded structures as stated in [1], wherein the steel comprises, based on weight, 0.01 to 0.2% of C, 0.01 to 0.4% of Si,
Chiba Toshihiko
Fuji Masao
Kasuya Tadashi
Kobayashi Jun'ichi
Ohkita Shigeru
Kenyon & Kenyon
Nippin Steel Corporation
Yee Deborah
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