Trailing edge stress relief process (TESR) for welds

Details Trailing edge stress relief process (TESR) for welds Trailing edge stress relief process (TESR) for welds

1999-10-13

2001-05-01

Ryan, Patrick (Department: 1725)

Metal fusion bonding

Process

With subsequent treating other than heating of bonded parts...

C228S001100, C228S110100

Reexamination Certificate

active

06223974

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention is related to producing welds with reduced residual stress level and without commonly known defects. More particularly invention relates to an improvement in the process for prevention of build up of residual stress disclosed in applicant's U.S. Pat. No. 4,386,727, Jun. 7, 1983.
The process described in applicant's patent mentioned above has received acclaim from scientific circles. But improvements are necessary and possible on the basis of further understanding of the welding process itself, the manner in which structures of microstructure are formed and causes how stresses come into effect in a weld.
2. Background
It is obvious that molten metal solidifies to form a weld nugget and it also makes joint/s between the interface of liquid metal and the solid face of the cup formed in base metals. Welding itself is a quasi-stationary state, in which a hill of heat moves over stationary weld plates arranged in juxta position for welding. Such a hill of heat is with set physical dimensions once welding attains a stable condition, as first observed by Rosenthal in 30's, and it moves along the center line of the weld at the welding speed.
As disclosed in my U.S. Pat. No. 4,386,727 the external stress of low value, applied at low frequency at center of weld nugget at one end, in experimental welds, prevents build up of residual stress during solidification. The stress required is of low value, because bond strength at solidification temperatures is low. Process in U.S. Pat. No. 3,741,820 and its improvement, U.S. Pat. No. 4,968,359, attempts application of frequencies close to natural mechanical resonant harmonic or its subharmonic frequencies of the entire weldment after weld solidification. These generate high stresses. A weld usually occupies only a very small fraction of volume of a weldment. Therefore major portion of energy developed is used in larger volume of the weldment; and only marginal benefits may be achieved in reducing residual stress in solidified weld. Further, such stresses do not act on individual crystals, but on solidified (monolithic) mass of crystals which is the weld nugget, along with other part of the weldment. Further the microstructure in the nugget is then at a temperature far lower than the solidification temperature and therefore bond strengths are high. These facts cause only marginal success in attempts to reduce residual stress in welds. Results of such processes are normally compared with the widely used thermal stress relief process which gives better results in reduction of residual stress. Another valid objection is that in such processes the useful fatigue life of the weldment is used up and therefore reduced.
In the process disclosed here, stresses are applied at the solidification temperature, to the Trailing Edge of the Weld (TEW), in particular to solid surface resulting from preceding Solidifying Metal Zone in the Second order Quasi Stationary (SQS) state; further as bond strengths are lower, input of energy at lower stress amplitudes is necessary. Further, stress applied is transmitted to individual growing crystals in the Liquid Metal Zone (LMZ) through semi-liquid state, which has a certain degree of mobility. These conditions should be compared with conditions when only a fraction of the applied stress that may be reaching the solidified mass of the weld when stress frequencies closer to harmonic or sub-harmonic are applied to the entire weldment. Minor positive effects on welds are observed by application of such processes during welding. These can be inferred from results which are embodiment of U.S. Pat. No. 4,386,727.
Modification of the microstructure and therefore reduction of residual stress and elimination of the discontinuities such as porosity, are distinct benefits of the process disclosed herein; therefore, the instant invention is a necessary improvement over all the methods, known so far.
BRIEF SUMMARY OF THE INVENTION
Quasi-stationary state in welding operation results in causing second order quasi stationary states to come into effect. In particular, the SQS state at the TEW is the most important as defects and residual stresses are built therein in the microstructure during the process of solidification and remain in weld for its life, unless steps are taken to modify structures in microstructures. Weld defects such as porosity once formed can never be eliminated.
It is known that (micro-) strcuctures are developed in atoms, molecules and their groups at sub-microscopic, microscopic and macroscopic level respectively during the process of solidification. Rapid heat conduction is one of the causes for overall stress in structural bonds to be in excess than required. Such retained stress in solidified microstructure is the residual stress.
In brief what I found is modification of structures in microstructure is possible and can reduce retained residual stress. In the Trailing Edge Stress Relief (TESR) process, it is reduced by input of external energy by application of the cyclic stresses to TEW in SQS state of a weld.
It is the principal object of my invention to develop a process which can be used with a fusion welding process like Gas Metal Arc Welding (GMAW), or Sub-merged Arc Welding (SAW), to produce a weld with reduced residual stresses.
Another object of my invention is to develop a process that produces welds with reduced discontinuities such as porosity.
Another object of my invention is to develop a process that can be applied during welding at the TEW in SQS state.
A still further object of my invention is to provide a process which is controllable externally.
A still further object of my invention is to provide a process so that parameters of frequency and stress amplitude appropriate for reduction of residual stress and weld defects for a specific alloy composition and and specific weld size, can be determined.
Another object of my invention is to make the stress relieving process user friendly, making the process part of the welding power set operation;
Further object of my invention is to provide a process that can be used conveniently for welding structures such as high rise steel structures or ships or submarines or welding under water or extended applications in inclement climate or in space, for which furnace treatment facilities can not be created.
Another object of my invention is to provide a process which will reduce non destructive testing and industrial handling and time delays due also to subsequent stress relieving operations where possible, such as post weld thermal treatments, along with costs associated with it. Further object of the invention is to reduce the handling of detached equipment such as vibrators or furnaces to stress relieve the weldment.
I found that these and other objects may be attained by a process in which stress cycles are applied to solidified weld metal surface following the SMZ, as the said SMZ being.
More particularly these objects may be achieved by application of appropriate stress amplitude/s and frequency/cies to the TEW as the welding gun moves at the welding speed, on the center line of weld. Further, when sufficient data on parameters of the TESR process is collected on several weldable alloys, the data base will be useful for designing residual stress level in a weld.
Further objects and features of my invention will be apparent from the following specifications and claims when considered in connection with drawings illustrating several embodiments of my invention.


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