Journal of Materials Processing Technology 74 (1998) 259–262

Research on residual-stress reduction by strong pulsedmagnetic treatment

A.L. Lu *, F. Tang, X.J. Luo, J.F. Mei, H.Z. FangDepartment of Mechanical Engineering, Tsing Hua Uni6ersity, Beijing 100084, People’s Republic of China

Received 26 December 1996

Abstract

Pulsed magnetic treatment is an entirely new method of reducing residual stress without heating. The tests show that properstrong pulsed magnetic treatment can lead to stress relaxation in both rigid restrained tensile specimens and welded specimens.The longitudinal and the transverse residual-stress distributions in the welded specimens indicate a general reduction. In addition,the magnetic treatment cycles have an influence on the ratio of stress reduction. © 1998 Elsevier Science S.A.

Keywords: Residual stresses; Magnetic treatment; Welding; Magnetic vibration; Stress relaxation

1. Introduction

In the course of mechanical manufacturing, resid-ual-stress is often introduced into materials by manyprocesses. In general, there are two kinds of residual-stress, one is caused by forced assembly when somecomponents in a structure are not coordinated wellwith each other, the other one is self-equilibratedresidual-stress caused by non-uniform elastic andplastic strain in different areas of the material whensubjected to cold or hot processing. In welded struc-tures, the latter is prevalent because of highly local-ized heat input and severely non-uniform strain.

It is known that the residual-stress is very harmfulto the performance and the service life of manyproducts. Therefore, in order to reduce or relieve theresidual-stress, various methods have been developed,which include manufacturing process optimization,post-weld heat treatment (PWHT), vibratory treat-ment, explosive treatment and cold rolling afterwelding, etc. All of these methods have some limits.The welding residual-stress in some large structures,

such as pressure vessels and the dipper handles ofexcavators, is ordinarily eliminated by PWHT at thepresent time. As a result, some side effects are pro-duced frequently. First, the microstructures of thematerials can get worse, and hence some propertydegradation may take place: this is particularly truefor high-strength steel. Second, most of the structuresare very large and their heat treatment is a difficulttask and involves a large consumption of time andenergy. Thus, it is of much significance to find asimple method that not only can reduce residual-stresses but also does not make the properties of thematerials worse. With better understanding of thephysical and mechanical properties of the materials,a new residual-stress reducing method, pulsed mag-netic treatment, was put forward. The test resultsshow that this method can reduce residual-stressesand has great potential [1].

Some practical applications of magnetic treatmentwere done in the United States. Some kinds of ma-chining tools were treated, their service performanceand life being much increased. A few scholars stud-ied the phenomenon and gave some tentative expla-nation, but quantitative results of residual-stress reliefby magnetic treatment have not been presented [2].This paper mainly introduces the strong pulsed mag-netic treatment on tensile specimens and weldedspecimens, the effects of magnetic treatment on resid-ual-stresses being studied.* Corresponding author. E-mail: lal@mail.tsinghua.edu.cn

0924-0136/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.

PII S 0 924 -0136 (97 )00280 -X

A.L. Lu et al. / Journal of Materials Processing Technology 74 (1998) 259–262260

Fig. 1. Rigid restrained tensile specimen and the arrangement of thepoints for stress measurement.

Fig. 3. Welded specimen and sections for residual-stress measurementand magnetic treatment.

2. Experimental procedure

A rigid restrained tensile specimen made of lowcarbon steel is shown in Fig. 1. The specimen is locatedon two columns on the base (shown in Fig. 2). Tensilespecimens at different stress levels can be secured bychanging the distance of the two holes in the specimen.

A restrained welded specimen with a double-V buttweld made of HT50 steel is shown in Fig. 3 (430×365×32 mm). First, two restraint welds each of 90 mmlength are made at both ends, respectively. Then themain weld in the middle is finished.

The blind-hole method is used to measure the resid-ual-stress, the arrangement of the points for measure-ment on the tensile specimen being shown in Fig. 1.Points 1 and 2 are measured before treating and points3 and 4 are measured after treating. Then the values oftwo points at the same transverse section are comparedwith each other. In restrained welded specimens theinitial residual-stresses are measured at sections markedas a and b (shown in Fig. 2). Magnetic treatment isdone at areas that are exactly symmetrical with areas towhich sections a and b belong. The section b% wastreated once whilst section a% was treated several times.The residual-stress was measured after magnetic treat-ment, the results of the measurement being comparedwith the values of the initial residual-stress.

High-powered discharge apparatus is used to pro-duce a make strong pulsed current by dischargingthrough different types of magnetizer, the strong pulsedmagnetic field obtained being used to treat the speci-mens. The intensity of pulsed magnetic field is up to 106

A m−1.

3. Experimental results

The experimental results for rigid restrained speci-mens are shown in Fig. 4. When the initial residual-stress level is in the range of 100–290 MPa, a generalreduction of 30–60 MPa can be obtained, with a

maximum reduction of 63 MPa after magnetic treat-ment.

The curves in Fig. 5a, b are the distributions of theinitial transverse and longitudinal residual-stress in therestrained welded specimens. In the sections a and b,the longitudinal stress indicates a small change, whilstthe change of the transverse stress is relatively large.The transverse stress is much larger than the longitudi-nal stress in areas some distance away from the weldbead. Thus, in the weld the longitudinal stresses aredominant, although in areas away from the weld beadthere are mainly transverse stresses. The comparison ofthe residual-stresses after one-pulse magnetic treatmentwith the initial stresses is shown in Fig. 6a, b, where thelongitudinal and transverse stresses all are reduced tosome degree, the average magnitude of reduction being�50 MPa. The residual-stresses after five-times pulsedmagnetic treatment are compared with the initialstresses in Fig. 7a, b. The average magnitudes of reduc-tion of the longitudinal and transverse stresses are �40and 90 MPa, respectively. At a distance of �40 mmaway from the weld bead, the magnitude of reduction is�100–150 MPa, the reduction being \40%. From the

Fig. 4. Comparison of the tensile stress in rigid restrained specimensbefore and after the strong pulsed magnetic treatment.Fig. 2. Assembly of the tensile specimen and the base.

A.L. Lu et al. / Journal of Materials Processing Technology 74 (1998) 259–262 261

Fig. 5. Showing the initial: (a) longitudinal; and (b) transverse;residual-stress distributions of sections a and b in the welded speci-mens.

persively in the metal and result in the stress relaxation.In a strong pulsed magnetic field, the intense dynamiceffect of magnetostriction must occur. As for iron,positive magnetostriction occurs in a weak magneticfield and negative magnetostriction occurs in a strongfield [3]. Therefore, with the continuous variation of theintensity of the strong magnetic field, alternating mag-netostriction take place and lead to internal magneticvibration in the specimen. This phenomenon may alsocontribute to the stress reduction. Consequently, it canbe deduced that the stress states of micro-areas in thematerial could be changed in a strong magnetic field, sothat along with the magnetic vibration, the stresses ofall levels are reduced. In addition, stress relaxationcaused by magnetic treatment is probably induced by adispersively distributed micro-plastic strain and not bya whole macro-plastic strain in the specimens. Thus, thecharacteristic of stress reduction is that the stress distri-butions are reduced generally by similar magnitudes

Fig. 6. Comparison of: (a) longitudinal; and (b) transverse; residual-stress distributions of sections b and b% in welded specimens beforeand after a once-only magnetic treatment.

above results, it can be found that the feature ofresidual-stress reduction by magnetic treatment is ageneral reduction in their distributions.

4. Discussion

Although the effect of magnetic treatment on resid-ual-stresses has been established by experiment, itsmechanism is not yet clear and the factors that influ-ence the effect are very complex, so that they have to beexplained from basic theories, tentatively so far. Atfirst, when magnetized, motions of domain wall andturning of magnetic torque of domain take place.Simultaneously domain walls surmount barriers orleave the pinning centers, then merge and disappear.The above processes may lead to the variation of thestress states of micro areas in the material and increasethe plasticity. Hence, plastic strains are promoted dis-

A.L. Lu et al. / Journal of Materials Processing Technology 74 (1998) 259–262262

Fig. 7. Comparison of: (a) longitudinal; and (b) transverse; residual-stress distributions of sections a and a% in welded specimens beforeand after multiple (five times) strong pulsed magnetic treatment.

and not only the relief of the peak. The residual-stressreducing mechanism proposed above should be studiedfurther in detail.

5. Conclusions

It has been established by tests that the residual-stress in both rigid restrained tensile specimens andwelded specimens can be reduced by strong pulsedmagnetic treatment. The longitudinal and transversestress distributions in welded specimens indicate thefeature of general reduction, but the magnitudes of thereduction are not the same in different areas. In addi-tion, residual-stress reduction by multiple treatments isbetter than that by a once-only treatment. Taking intoaccount the mechanism of residual-stress reduction andthe optimization of parameters of the magnetic treat-ment, the corresponding experiments are only at theinitial stages.

Acknowledgements

The work was supported by the National NaturalScience Foundation of the People’s Republic of China.

References

[1] X.J. Luo, Research on residual stress relieving in welded struc-ture with magnetic field, M.Sc. Dissertation, Tsing Hua Univer-sity, Beijing, 1995, (in Chinese).

[2] R.F. Hochman, N. Tselesin, V. Dris, Magnetic Fields: FertileGround for Metals Processing, Advanced Materials and Pro-cesses Inc., Metal Progress, (8) (1988) 36.

[3] W.D. Zhong, Ferromagnetics, vol. 2, Science Publishing House,Beijing, 1987, p. 28.

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