Advances in Materials Science and Engineering
Volume 2018 (2018), Article ID 4826020, 5 pages
https://doi.org/10.1155/2018/4826020

Research Article

Evaluation of Residual Stress on Steel Parts Welded and Milled

Frederico Ozanan Neves, Antônio Célio Moreira Junior, and Alex Sander Chaves da Silva

Laboratório de Fabricação, DEMEC/UFSJ, Praça Frei Orlando 170, São João del Rei, MG, Brazil

Correspondence should be addressed to Frederico Ozanan Neves; fred@ufsj.edu.br

Received 20 October 2017; Revised 24 December 2017; Accepted 8 January 2018; Published 19 March 2018

Academic Editor: Fernando Lusquiños

Copyright © 2018 Frederico Ozanan Neves et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Welded parts are common in mechanical engineering. As all manufactured parts, they also present residual stresses introduced by the corresponding manufacturing process. Residual stresses can be beneficial or not because they can increase or reduce the useful life of the mechanical components, particularly when they are subjected to a cyclic stress in which they can fail by fatigue. In this study, SAE 1045 steel samples were welded by metal inert gas process, varying the speed and welding current. The welded samples were thereafter milled, including the welded region. Residual stresses on material as received, welded, and welded and subsequently milled were evaluated through the microhardness method. A factorial statistical design was used, and the results were studied by analysis of variance. It can be concluded that, in general, welding introduces compressive residual stresses which are improved by posterior milling operation, and there is an optimal set of operating parameters for this condition.

1. Introduction

Currently, it is necessary to understand and control the manufacturing processes and manufactured products to achieve higher efficiency and production quality as well as low-cost operations. The gas metal arc welding (GMAW) is a procedure considered as advantageous when compared to other welding processes due to production capacity, applicability, and the automation possibilities. Several parameters can influence a weld, which make adjustment procedure difficult [1, 2]. Among them, welding electric current, the polarity of the arc, the composition and the diameter of the electrode, shielding gas, and welding speed are the main parameters [3].

Welding current is very important in the process and must be adequate to the wire speed feed [4]. Welding current is set up from information about the material to be welded, the electrode type and its diameter, and the protection gas. Another important parameter is welding speed. A common strategy is to increase welding current as welding speed is increased, normally in a proportional way. If the welding current or welding speed is above a critical value, a defect named humping occurs. In this case, the welding current combined with welding speed represents the upper limit to design, that is, the upper limit of welding current and welding speed combination in parameter optimization design [5]. When mechanical and thermal processes are realized, residual stresses can be generated. In welding, tensile residual stresses occur as result of the high heat input, which reduce fatigue strength [6]. Residual stress is induced by machining processes and, depending on its magnitude and sign, can be very detrimental to mechanical parts when they are in service [7]. Several studies are being realized to diagnose the type and nature of these stresses originated from machining [8–10].

Residual stress can be evaluated using indentation tests. This method compares the surface hardness of a material before and after manufacturing process. If the hardness is higher than the hardness of the material after processing, it indicates that a compressive residual stress is present on its surface. If lesser, then tensile residual stresses are present [11–13]. In this study, a milled operation is done after a gas metal arc welded, to increase the surface finishing of the welded part. Then, residual stress is evaluated before and after processes to recommend or not these combined operations.

2. Materials and Methods

The experiment was done using a randomized factorial design. The influence variables were welding current, welding speed, the regions of the part, and two process conditions: parts only welded and parts welded and subsequently milled. The response variable was the microhardness, associated with residual stress. Welding was done in plates of 100 × 100 × 3.2 mm of SAE 1045 steel.

The plates were wet grinded and cut into two parts with a closed joint. An AWS ER-70S-6 electrode with a diameter of 0.8 mm was used. Protection gas used was constituted of 80% of argon and 20% of CO₂. This gas mixture was used as recommended by Moyer [14] and Suban and Tusek [15] to increase productivity and hardness of the bead weld.

After tests, three levels were chosen for welding current: I1 = 140 A, I2 = 160 A, and I3 = 176 A.

Three weld speeds were adopted: V1 = 4.5, V2 = 5.4, and V3 = 6.4 (mm/s). Three regions were observed in the welded part, as can be seen in Figure 1: (MZ) metal zone, (AZ) affected zone, and (BM) base material.

Figure 1: Affected regions after welding.

A group of welded parts were milled to evaluate the residual stress in this condition to be compared with simple welded part. The parameters used in milling were depth of cut (d) = 0.3 mm, cutting speed (Sc) = 250 m/min, and feed rate (f) = 100 mm/min. The mill used was a tool with 2 teeth. Microhardness Vickers test was done using a load of 5 N in 20 s. Six points (P1, P2, P3, P4, P5, and P6), three in each side of the bead weld, were fixed to measure the microhardness, as shown in Figure 2.

Figure 2: Measurement position (dimensions in mm).

3. Results and Discussion

An indentation test was done in the material as received to be compared with the parts welded and parts welded and milled. The results are shown in Table 1.

Table 1: Microhardness Vickers of the material as received by position of measurement.

P1
P2
P3
P4
P5
P6

Replica 1
238
242
240
240
242
238

Replica 2
239
243
241
240
242
239

Replica 3
240
242
240
241
243
240

In Table 2, the results of Microhardness Vickers obtained from parts welded and from parts welded and milled are shown. In Table 2, V1 = 4.5 m/min, V2 = 5.4 m/min, V3 = 6.4 m/min, I1 = 140 A, I2 = 160 A, and I3 = 176 A.

Table 2: Microhardness of parts welded and parts welded and milled.

Parts welded

I1
I2
I3

P1
P2
P3
P4
P5
P6
P1
P2
P3
P4
P5
P6
P1
P2
P3
P4
P5
P6

V1
245
250
255
256
250
246
250
252
253
252
250
256
254
254
257
255
256
254

248
253
255
255
249
248
247
253
255
249
248
248
252
253
256
260
259
258

247
253
256
256
250
249
250
252
254
255
250
248
251
253
258
258
256
253

V2
248
251
254
254
249
247
249
249
254
253
251
250
259
261
260
257
252
250

251
253
257
257
248
252
250
249
252
254
254
253
253
260
255
254
250
252

248
254
257
257
254
245
252
253
255
256
255
254
252
255
256
254
253
251

V3
250
252
254
254
250
250
252
250
254
253
251
251
253
252
258
259
258
251

252
253
254
254
249
249
249
249
252
255
253
255
252
255
256
257
258
252

249
250
252
252
250
248
253
255
257
254
255
250
255
258
260
259
258
255

Parts welded and milled

V1
276
280
286
286
270
260
270
273
279
282
280
269
268
290
284
270
268
270

278
272
275
276
258
268
269
270
270
279
270
269
260
269
274
290
286
279

261
269
270
280
271
266
262
270
275
277
269
263
267
271
280
274
270
273

V2
255
264
268
271
277
275
275
276
280
282
281
271
273
279
273
275
268
270

254
257
264
270
269
269
272
272
275
280
278
270
276
280
274
285
280
280

279
271
280
263
270
267
275
276
280
280
278
271
292
282
280
290
280
276

V3
270
274
279
280
270
270
266
286
283
269
265
272
290
290
294
300
293
280

278
270
267
275
270
269
256
249
268
271
258
270
298
295
299
295
293
275

270
271
273
275
274
270
246
270
278
265
260
270
290
284
295
299
293
280

An analysis of variance (ANOVA) with 95% of confidence was used. First, the results were analyzed for the parts only welded. Table 3 shows the results of the ANOVA, where SS = square sum, DF = degrees of freedom, MSS = mean square sum, and value is the significance. When value is lesser than 0.05, the result is significant, and the variable has influence in the process. In this table, the principal effects are V (welding speed), I (welding current), and P (position of the measure). VI, VP, and IP are the double interactions between the principal effects, and VIP is the triple interaction of the principal effects.

Table 3: Analysis of variance of the welded parts.

SS
DF
MSS
value

Total
1824.698
161
—
—
—
—

V
12.308
2
6.15
1.503
0.227
—

I
461.086
2
230.543
56.331
0.001
Significant

P
538.623
5
107.725
26.321
0.001
Significant

VI
30.246
4
7.561
1.847
0.125
—

VP
46.876
10
4.687
1.145
0.336
—

IP
106.765
10
10.67
2.608
0.007
Significant

VIP
186.791
20
9.339
2.282
0.004
Significant

ERR
442.000
108
4.092
—
—
—

From the ANOVA, it can be concluded that welding current and the measurement position are significant and influence the microhardness. The welding speed, however, proved to be not significant and does not influence the hardness of the parts.

The analysis of variance also showed an interaction between the welding current and the measurement position, and there was also interaction between the three variables. An orthogonal contrast test revealed that there is no difference between the results obtained with the welding current I1 and I2. But, the hardness increases when the welding current I3 is used. As the increasing of hardness is associated with the introduction of compressive residual stress, we can conclude that welding current at more high level is beneficial to fatigue strength in welded parts.

The measurement position also showed to be different. The position P1 is symmetric to position P6, as well as position P2 and P5, and position P3 is symmetric to P4. An orthogonal contrast test revealed that there is no difference between the symmetric positions, as we expected. But there is a difference between the hardness measured in the position in a same side. The hardness in the positions P3 and P4 are higher than hardness in the positions P2 and P5, and these are higher than measurements at positions P1 and P6. So, we can conclude that hardness decreases from the region near the bead weld toward the edge of the part.

The analysis of variance to the welded and milled condition is presented in Table 4.

Table 4: Analysis of variance of the welded and milled parts.

SS
DF
MSS
value

Total
15,222.94
161
—
—
—
—

V
421.800
2
210.907
5.394
0.006
Significant

I
3735.800
2
1867.910
47.774
0.001
Significant

P
1479.900
5
295.981
7.570
0.001
Significant

VI
3361.200
4
840.315
21.492
0.001
Significant

VP
291.666
10
29.166
0.745
0.680
—

IP
210.555
10
21.055
0.538
0.859
—

VIP
1499.259
20
74.963
1.917
0.018
Significant

ERR
4222.667
108
39.098
—
—
—

After milled, the three variables are influents. There is interaction between the welding current and welding speed, and there is interaction between the three variables. An orthogonal contrast test revealed that there is no difference between the results obtained with the welding speeds V1 and V2. But, the hardness increases when the welding speed V3 is used. In the same way, the welding current presents no difference between I1 and I2, but the welding current I3 influences the microhardness on welding part. As it can be seen in the welded part, there is no difference between the symmetric positions. But there is difference between the hardness measured at positions in the same side. The hardness in the positions P3 and P4 is higher than hardness at positions P2 and P5, and these are higher than measurements at positions P1 and P6.

Figure 3 presents the curves of microhardness measured to the welded and welded and milled and as-received parts as a function of measurement position and welding current, when the welding speed is the intermediated speed V2 = 5.4 mm/s. The same behavior is verified with the other two welding speeds. In Figure 3, it can be noted that the welding increases the microhardness compared to that measured in the part as received. Similarly, milling also increases the microhardness over only the part welded. Also, the behavior of the microhardness is symmetrical in relation to the weld bead and shows greater from central positions (P3 and P4) toward the edge of the piece (P1 and P6), regardless of processing (welded or welded and milled) and the welding current. It is important to pay attention to the fact that the effect of welding current be more pronounced in the welded and milled part.

Figure 3: Microhardness for welding current and measurement position: effect of welding current.

In Figure 4, the curves of microhardness to the welded and welded and milled are presented and as received parts as a function of the measurement positions to welding speed. In this case, the values are those obtained when the welding current is the intermediated current I2 = 160 A. The same behavior is verified with the other two welding currents, as shown in Figure 3. As it was seen from welding current, welding speed also increased the microhardness of the part compared with the part as received. And by milling the welded part, the microhardness increased more. But, in a different way, the effect of welding speed V3 was not so pronounced in the microhardness as the effect of welding current I3, as seen in Figure 3.

Figure 4: Microhardness for welding current and measurement position: effect of welding speed.

Here, the distribution of microhardness in surface of the parts of the welding speed variable, verified in the points P1 to P6, is equal to that of welding current variable.

4. Conclusions

As demonstrated by other authors, the hardness can be associated with residual stress. If it increases, the presence of compressive stress is noted. Other way, if it decreases, there is the presence of tensile stresses. In the case of welding of a SAE 1045 steel, it can be observed that compressive residual stresses are introduced. The intensities of these residual stresses are greater near the bead weld and decrease toward the edge of the part. Welding current increases still more the intensity of the residual stresses as detected in the welding current of 176 A. The effect of welding speed on residual stresses is lesser than the effect of welding current. The milling operation in a part after welding proved to increase the compressive residual stresses. In this case, the effect of the welding current of 176 A was very important. So, as a general conclusion, considering that a presence of a compressive residual stress is beneficial to fatigue strength and that the milling improves the surface quality of the product in relation to the welded surface, this work recommends the milling operation after welding of this kind of parts.

Conflicts of Interest

The authors declare that they have no conflicts of interest regarding the publication of this paper.

Acknowledgments

The authors want to thank FAPEMIG–Fundação de Amparo a Pesquisa do Estado de Minas Gerais for the financial support to this research.

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