Evaluating the Effectiveness of the Temper Bead Welding Technique in AISI 4140 Steel Using GMAW with Electrode ER4130

Abstract

Abstract  This study evaluated the effectiveness of the Temper Bead Welding (TBW) technique in AISI 4140 steel using GMAW process and an ER4130 filler metal. The selection of welding parameters was first obtained using the Higuchi test, where welds were applied with several heat inputs. Three test coupons were welded using three different welding techniques: (1) conventional build-up in As-Welded (AW) condition, (2) conventional build-up welding with post-weld heat treatment (PW), and (3) the Temper Bead or Layer (TB) technique. The effectiveness of the TBW technique was evaluated by nondestructive examinations, carrying out a metallographic study of the microstructures, measuring the typical microhardness profiles of the weld metal and Heat-Affected Zones (HAZs). Tensile tests were also conducted with all-weld-metal specimens extracted from the coupons. Visual and radiographic examinations showed that the filler metal, ER4130, is highly susceptible to porosity. According to the results obtained, TBW has a more beneficial effect on the hardness of the cap beads than the build-up technique in the AW condition. The results also indicate that the hardness values of the HAZs in welds applied by TBW tend to be like those obtained in welds subjected to tempering heat treatment. This fact offers an economic advantage in situations where Post-Weld Heat Treatment (PWHT) cannot be performed.

Key-words:
Temper Bead Welding technique; Build-up; As-welded condition; Post-Weld Heat Treatment (PWHT); HTLA

1. Introduction

AISI-SAE 4130, 4140 and 4150 steels belong to a family of Heat-Treatable Low-Alloy (HTLA) steels alloyed with medium carbon plus chromium and molybdenum; hence, they are often called “chromoly steels”. The microstructure and mechanical properties of these steels are achieved after quenching and tempering, called Q&T [¹], [²]. These Cr-Mo steels are used in the automotive and aerospace industries [³]-[⁵], heavy machinery (dies, shafts, gears) [⁶], and large construction projects in the oil industry. Multiple components for drilling facilities in offshore rigs for oil and gas extraction —such as cement and sludge systems, wellheads, Christmas trees, heads, flanges, and piping accessories— are all built with chromium-molybdenum 4130 and 4140 steels instead carbon and low alloy steels [⁷]. The reason for this is that a medium carbon content plus additions of chromium and molybdenum in 4130 and 4140 steels (depending of the heat treatment) help to achieve tensile strengths in excess of 774 MPa, which is appropriate to withstand the high pressures required during drilling operations (10-15 ksi or 69-100 MPa) [⁸] with lower thicknesses than those required for carbon and High-Strength Low-Alloy (HSLA) steels.

During the construction, repair, and maintenance of equipment for drilling facilities, onshore and offshore contractors must conduct welding operations using conventional processes such as Shielded Metal Arc Welding (SMAW), Gas Metal Arc Welding (GMAW), Gas Tungsten Arc Welding (GTAW), and Submerged Arc Welding (SAW). Among the welding processes available to weld 4130 and 4140 steels, GMAW is widely used in the industrial sector because it has some technical and economic advantages over other joining processes like SMAW or GTAW.

AISI-SAE 4130 and 4140 steels have been welded with low-carbon filler metals that are usually high-strength low-alloys, such as ER70S-D2 [⁹], E8018-D2 [⁸], ER80S-D2 [³], [⁸], E8018-B2 [¹⁰], E9018-B3 H4 [⁷], E10018-D2, and E10018-M [⁸]. Some studies have used even low-strength carbon steel fillers such as ER70S-2 [¹¹]. All these filler metals produce undermatching welded joints with mechanical properties lower than those of 4130 and 4140 steels. The main advantage of undermatching weld metals is that they reduce the susceptibility to Hydrogen-Induced Cracking (HIC) [¹²]. To obtain matching properties after quenching and tempering, some manufacturers (i.e., Lincoln, Midalloy, and Harris) produce alloyed filler metals such as ER4130 for GMAW and GTAW, which produces, as the name implies, undiluted weld metals with the same chemical composition as AISI-SAE 4130 steel. This filler metal can be produced according to military standards as Type I in MIL-R-5632 [¹³] or other SAE aerospace standards like SAE-AMS 6457 to match 4130 steel, and SAE-AMS 6452 for ER4140.

AISI-SAE 4130 and 4140 steels present two main weldability issues: (1) their susceptibility to HIC [¹⁴] and (2) the low toughness of their welds without PWHT due to their tendency to produce a very hard fresh martensite during the welding thermal cycle both in the HAZ and in homogeneous weld metals. In order to avoid brittle fracture in 4130 and 4140 weldments, a PWHT is recommended to relieve the residual stresses and temper the martensite [²], [¹⁰]. However, there could be economic and logistical constraints, and it is not always possible to conduct a PWHT after welding repairs of complex equipment —such as that used in the oil industry or in heavy machinery— because it is impractical and expensive for field repairs [¹⁵]. As a result, the Welding Procedure Specifications (WPSs) developed without PWHT have advantages in terms of repair time and costs [¹⁰], [¹⁶].

The need to avoid PWHTs has stimulated the research and development of welding techniques that promote both the self-tempering of martensitic microstructures and the refinement of the coarse grain zone. Perhaps the most commonly used among these techniques is the TBW which is defined as “[…] a weld bead placed at a specific location in or at the surface of a weld for the purpose of affecting the metallurgical properties of the heat-affected zone or previously deposited weld metal” [¹⁷:11]. This technique requires multi-pass welded joints so that the heat input from a weld bead or layer affects the HAZ and/or weld metal metallurgy from the previously deposited beads or layers [¹⁸]. TBW can be applied in different forms or methods, such as the half-bead technique [¹⁵], the two-layer technique [¹⁰], and the surface temper bead reinforcing layer [¹⁷], but all of them share the same fundamentals.

Researchers have shown great interest in studying and evaluating alternative techniques to avoid PWHT. Therefore, there is significant research on the advantages of TBW for multiple base metal alloys. A compendium of these techniques was published by the Welding Research Council (WRC) in Bulletin 412 [¹⁹]. Higuchi et al. published a study on the repair of low-alloy forged steels with SMAW and a carbon steel filler metal using the half-bead tempering technique. In that work, he established the experimental basis to define the metallurgical characteristics of several types of welding and their effectiveness in tempering the HAZ [¹⁵]. His method, classic in this kind of studies, is called the “Higuchi Test”. Alberry developed a computational model for steel ASTM A508 Class 2 to predict the hardness of the HAZ in multi-pass welds applied by GTAW and the half-bead technique [²⁰]. Bueno [²¹] investigated the development of a WPS for AISI-SAE 4140 steel without PWHT using SMAW, a E8018-B2 filler metal, and the Higuchi test to evaluate the most suitable heat inputs for tempering layers. Lant et al. [¹⁶] reviewed weld repair procedures for HSLA steels to minimize future cracking risks and discussed the half-bead and temper layer techniques. In a German-Japanese seminar, Mizuno et al. presented their study about the appropriate welding conditions for repairing SQV2A pressure vessel steel, which corresponds to ASTM A533 Type B Class 1; they used the GTAW process autogenously [²²]. Some years later, they published a more complete study with the same material and the temper bead technique [²³]. Silva et al. [¹⁰] studied the effect of the two-layer temper welding technique on the properties of the HAZ in 4140 steel using the SMAW process, E8018-B2, and the Higuchi test. Mohr et al. [²⁴] investigated the temper bead technique applied to the repair of amine towers with the SMAW process and an Inconel 182 filler metal to join Inconel 625 rings on SA516 tanks: their goal was to avoid PWHT while ensuring low hardness. Aloraier et al. [²⁵] reviewed welding techniques without PWHT (i.e., half-bead and temper bead techniques) to repair ferritic alloys and their effects on residual stresses [²⁵]. Meszaros et al. [²⁶] studied the effects of the temper bead welding technique on the integrity of ASTM A516 Grade 70 steel welds using SMAW and E7018-1 [²⁶]. Naranjo et al. [⁷] designed a welding procedure for 4130 steel without PWHT using SMAW and E9018-B3 H4. Later, Naranjo et al. [²⁷] published their results. Tomków et al. [²⁸] studied the effect of the TBW technique on thermo-mechanically rolled S460ML steel using SMAW with a rutile filler and Controlled Thermal Severity (CTS) tests.

All the studies reviewed above about the temper bead welding technique in 4130 and 4140 steels used heterogeneous welds and undermatching strength where the base metal hardenability is quite greater than that of the weld metal. Based on this review of the state of the art, it can be established that the effectiveness of the TBW technique has not been studied in GMAW using matching HTLA filler metals alloyed with Cr-Mo (such as ER4130) for welded joints on AISI-SAE 4140 steel as the base metal. Furthermore, no study so far has compared the mechanical and metallurgical properties of these matching welds in the AW condition with conventional built-up welds with and without PWHT. That is the main objective of the present study.

2. Experimental Method

This study was divided into two stages: (1) Higuchi tests and (2) Temper Bead/Layer Welding (TBW) tests using partial penetration V-joints. The test coupons were welded using a Miller Invision 456MP power source with a Miller 70 S-74D wire feeder. A Bug-O MDS-1005 oscillation control module was used to apply the Higuchi test welds in mechanized mode. The test coupons used to evaluate the temper layer technique were welded manually. Figure 1 shows the experimental setup, and Table 1 details the fixed welding materials and parameters established in the experimental stage.

Figure 1
Welding equipment and devices used during the study.

Table 1
Welding parameters established during the experimental stage.

FIXED PARAMETER	VALUE
Base material	AISI 4140 Q&T
	(BÖHLER V320 Q&T)
Plate thickness	19 mm (3/4”)
Filler metal classification	ER4130 (MIDALLOY)
Filler metal diameter	1.14 mm (0.045”)
Shielding gas	98% Ar + 2% O2
Nozzle diameter	12.7 & 16 mm (1/2 & 5/8”)
Gas flow rate	14 l/min (30 SFCH)
Current type and polarity	DCEP (Reverse polarity)
Minimum preheat temperature	250°C
Contact-tip-to-work distance	18 mm
Work angle/Filler metal angle	90°/0°

2.1. Higuchi test

The coupon for the Higuchi test was made of AISI 4140 steel (BÖHLER V320 Q&T steel) and had the following dimensions: 250 mm x 100 mm x 19 mm with four 8-mm deep 60° V-grooves, as shown in Figure 2. The test welds were applied into these grooves in a flat position (1G) with four different heat inputs according to the parameters shown in Table 2. The finished welds were examined with penetrating testing according to ASTM E165 [²⁹]. Then, cross sections were removed from each weld, polished with abrasive paper up to a #2000 grit, finished with 12.5 µm of alumina and 1 µm of diamond particles, and pickled with Nital 5 (100 ml ethanol + 5 ml HNO₃). Hardness profiles were measured at an orientation of 45° with respect to the surface of the coupon, according to ASTM E92 standard [³⁰], using an INDETEC ZHμ durometer with a 2-kg load for 10s. A microstructural analysis was conducted using a NIKON Eclipse reflected light optical microscope.

Figure 2
(A) Higuchi test coupon and (B) geometry and dimensions of the V-grooves.

Table 2
Welding parameters used during the Higuchi test.

Welded	Tp/TIP	Machine setup		Average current	Welding	Welding
test	(°C)				speed	heat input
coupon		WFS (ipm)	Voltage (V)	(A)	(ipm)	(kJ/in)	(kJ/cm)
H1	258	150	20	157.5	14.1	13.4	5.3
H2	260	250	24	225	14.1	23.0	9.1
H3	250	350	28	275	14.1	32.8	12.9
H4	254	400	30	305	14.1	38.9	15.3

2.2. Temper Bead/Layer Welding (TBW) tests

Three 250 mm x 102 mm x ~ 19mm BÖHLER V320 Q&T (AISI-SAE 4140) steel coupons were used to evaluate the TBW technique. The edges were beveled at 35° by milling and then buttered with a single layer applied with MIDALLOY ER4130. The full penetration butt joint was a modified AWS B-U2-GF joint designation with an included angle of 70° and a GULLCO 1G42-R KATBAK® Ceramic Weld Backing, as seen in Figure 3. Three test coupons were used to evaluate three techniques: (1) conventional weld with pass accumulation (build-up) in the as-welded condition, that is, without PWHT; (2) conventional build-up welding with PWHT; and (3) the TBW technique without PWHT. The welding parameters used for the three coupons were between those of Higuchi tests H2 and H3 in Table 2.

Figure 3
Test coupons with B-U2-GF grooves and ceramic weld backing. (A) Groove adapted from AWS D1.1 (B-U2GF designation); (B) Welding test coupon with 1G42-R ceramic weld backing.

After a visual inspection, the welded coupons were examined with two Non-Destructive Testing (NDTs) as follows: (1) penetrating testing according to ASTM E165 and (2) radiographic testing according to AWS D1.1 code [³¹], Clause 6, Part E (AWS D1.1, 2015). Different specimens were cut off from the coupons and mechanized to analyze their microstructure, hardness, side bending according to AWS B2.1, and tensile strength according to ASTM A370 [³²]. In order to compare the three welding techniques, the specimens were prepared for microstructural evaluation and the measurement of microhardness profiles. Such measurement was carried out following the general lines 1, 2, 3, and 4 shown in Figure 4, and in accordance with those required in Section IX of the ASME Boiler and Pressure Vessel Code (B&PV) for the qualification of WPSs that use the TBW technique (lines A1, A2, and A3). Profile 3 (P3) was taken with a direction approximately perpendicular to the fusion line with the idea of obtaining the true dimension of the heat affected zone (HAZ) across the face of the joint and reducing the possibility of the hardnesses being in a banded region that is parallel to the rolling direction.

Figure 4
Hardness profiles established according to ASME B&PV Code (A1, A2 and A3) and general profiles (1, 2, 3, and 4).

3. Results and Discussion

3.1. Chemical composition of the base and filler metals

Table 3 details three kinds of chemical compositions of the BÖHLER V320 steel and MIDALLOY ER4130 filler metal: typical composition, Mill Test Report (MTR) composition, and composition measured with a BRUKER Q8 MAGELLAN Optical Emission Spectrometer (OES). The undiluted chemical composition of the ER4130 filler metal was determined using a test coupon in accordance with AWS A5.28:2005. There is great similarity between the typical values of ER4130 and those values measured by OES, as well as between the MTR values issued by BÖHLER for the V320 steel and those measured by OES.

Table 3
Chemical composition of the base and filler metal (wt. %).

SOURCE	C	Cr	Mo	Mn	P	S	Si
ER4130 – typical composition	0.31	0.92	0.20	0.52	0.005	0.0064	0.186
ER4130 - composition measured by OES	0.283	0.925	0.202	0.467	0.0073	0.0064	0.186
V320 – composition in the MTR	0.400	1.040	0.210	0.910	0.013	0.016	0.260
V320 – composition measured by OES	0.402	1.013	0.21	0.954	0.015	0.017	0.271

3.2. Higuchi test

3.2.1. Macrographic analysis

Figure 5 shows the macrographs of the four welds taken from Higuchi coupons. Clearly, the size of the molten metal increases as the Heat Input (HI) is higher or, in other words, as the Wire Feed Speed (WFS) is increased, as shown in Table 2. At a HI of 5.3 kJ/cm, the shape of the molten metal has a low-penetration parabolic profile. At 12.9 kJ/cm and 15.3 kJ/cm, the weld metal penetration shape becomes finger type. The finger type profile is characteristic of GMAW welds with Argon-rich atmospheres that produce a non-homogenous arc energy distribution with a high-energy center surrounded by a low-energy outer ring [³³].

Figure 5
Macrographs of the four weld beads in the Higuchi test. (A) Heat input = 5.28 kJ/cm (H1); (B) Heat input = 9.05 kJ/cm (H2); (C) Heat input = 12.90 kJ/cm (H3); (D) Heat input = 15.33 kJ/cm (H4).

3.2.2. Microstructure and hardness of the HAZ and the weld metal

The combined results obtained from the weld macro-structures and all the hardness profiles showed that the HAZ of the V320 steel has three sub-regions: (1) a hard zone with an average hardness of 579 ±37 HV_2/10 corresponding to the total austenitized region with peak temperatures between A₃ (about 800 °C for 4140 steel according to the Fe-C diagram) and the melting temperature (about 1420 °C); (2) a zone with an abrupt hardness reduction that underwent transformations in the inter-critical range between A₁ (727 °C) and A₃; and (3) a soft zone with an average minimum hardness of 255 ±16 HV_2/10, which cannot be observed in the macrographs or in the microstructure obtained by optical microscopy. The fusion zones obtained with ER4130 presented an average hardness of 439 ±16 HV_2/10.

Figure 6 shows the macrograph, microhardness profile, indentations, and microstructure of the weld applied with a HI=15.3 kJ/cm. Different regions can be identified in the weld: the fusion zone, a hard HAZ near the fusion boundary (denoted here as HAZ_HARD), a hardness transition (inter-critical) region, the over-tempered soft HAZ (denoted here as HAZ_SOFT) not visible at macro or micro level, and the unaffected base metal. The microstructure of the four welds is very similar. It contains a fusion zone (composite zone) that is a mixture of V320 (AISI 4140) steel and the ER4130 filler metal with an average hardness of 439 ±16 HV_2/10, which corresponds to a content of fresh martensite between 60% and 70% —this is consistent with in the microstructures of all the fusion zones.

Figure 6
Macrograph, hardness profile, and microstructure of the weld applied with 15.3 kJ/cm (38.9 kJ/in).

The HAZ closest to the fusion line presents a sub-region harder than the original base metal, with a total average of 579 ± 37 HV_2/10. This sub-region underwent grain growth, total austenitization, and transformation to a mixture with 70-80% of fresh martensite, bainite, and possibly retained austenite. Next to the HAZ_HARD, there is another sub-region of partial austenitization that corresponds to the inter-critical range between A₁ (727 °C) and A₃. In this sub-region, the hardness changes from the highest values of the HAZ_HARD to lower values in the isotherms near A₁, and the amount of transformation to austenite is reduced. The microstructure of this sub-region presents banding like that of the base metal but much more evident, as can be seen in the micrography in Figure 7, which is a close-up of said sub-region in H4.

Figure 7
Microstructure of the H4 weld at HI=15.3 kJ/cm (38.9 kJ/in).

Finally, there is a sub-region of low hardness (255 ±16 HV_2/10 on average) that cannot be studied by optical metallography. It is the over-tempered region of the original martensite, in which there is no formation of new martensite because there was no austenitization. After this hardness reduction at the HAZ_SOFT, the hardness value increased gradually until it reached the original hardness of the base metal (i.e., 293 HV_2/10).

Both the V320 base metal and the HAZ_HARD show a microstructure composed of bright and dark bands formed by segregation during the steel rolling process, following the same direction of the plate rolling as shown in Figure 7. According to some researchers who studied 4140 steel, bright bands have higher chromium and carbon contents (1.2% Cr) than dark bands (0.9% Cr), which makes them very hard after tempering [³⁴]. The segregation of Cr and C in the bright bands, in addition to a fast cooling of the fully austenitized HAZ adjacent to the fusion boundary, produces a larger amount of martensite due to its higher hardenability and also higher hardness thanks to a higher carbon content in this region [³⁴]. This explains the high hardness values found in the bright bands in Figure 6, where white areas correspond to hardness peaks, in contrast with the low values in dark areas (all showed with red arrows).

3.2.3. Conclusions drawn from the results of the Higuchi test

The results of the Higuchi test indicated that two relevant considerations should be taken into account for the TBW technique: (1) avoid using a low-heat input (5.3 kJ/cm or 13.4 kJ/in) because it produces low penetration and a very low width in the HAZ_SOFT; and (2) avoid using a high-energy input (15.3 kJ/cm or 38.9 kJ/in) because the finger type shape of the fusion zone affects the uniformity of the microstructural modification intended by the TBW technique. Therefore, average heat inputs between 9.1 kJ/cm and 12.90 kJ/cm were selected for this study.

3.3. Comparison of welding techniques: AW, PW, and TB

This subsection compares the results of the NDTs and destructive testing (macro etching, microstructure, hardness, and tension) applied to the welded coupons produced by three techniques: build-up As-Welded (AW), build-up + PWHT (PW), and Temper Bead/Layer (TB) welding. Figure 8 shows the sequence in which the passes and layers were applied on the TB coupon. Each weld bead is coded as B#, where B stands for “bead”, and # is the number of the weld bead (e.g., B5 corresponds to Bead 5).

Figure 8
Sequence of application of the weld layers and beads on the TB coupon according to the Section IX of ASME B&PV Code.

3.3.1. Nondestructive examination

Figure 9 shows the radiographs of the three test coupons obtained by the three welding techniques: AW, PW, and TB. Remarkably, there is much porosity that could not be avoided with any set of welding parameters, not even with a thorough cleaning of the joint and between passes. The ER4130 filler metal exhibits a noticeable susceptibility to producing porosity, which may be due to the low amount of deoxidants, such as Si (0.186% - Table 3).

Figure 9
Radiographs of the welded coupons. (A) AW; (B) PW; (C) TB.

Oxygen in the weld pool due to the shielding gas (98%Ar-2%O₂) can react with carbon of weld metal (0.283%C according to Table 3) to form CO₂ during solidification, which can result in porosity [¹]. Other sources of porosity in GMAW may be associated with “hydrogen-induced porosity” caused by lubricants necessary for cold drawing the wire or, even, uncontrolled moisture content in the shielding gas which tends to rise when the cylinder is emptied [³⁵].

3.3.2. Macrostructure examination

Figure 10 shows the macrostructure of the joint welded with the TBW technique extracted from the TB coupon and the hardness indentations for Profiles 1, 2, 3, 4 (perpendicular to the fusion line), and ASME Profiles A1, A2, and A3. The numbers correspond to the HAZs of the different welding beads (e.g., 2 is the HAZ of B2). A semicircular lack of fusion was detected in the lower part of B14, and some porosity was observed in the last bead (B20). The lack of fusion (Figure 10) may be due to the finger-tip shape of the weld metal associated with argon rich atmosphere. In general, most of the molten metal area was affected by the multiple thermal cycles. However, some unaffected weld metal areas, which can be identified in Figure 10B, maintained their solidification microstructure, such as some regions of beads B2, B9, B11, B14, B18, B19, and, of course, the whole area of B20 (i.e., the last pass applied in this weld). The total amount of molten metal modified or “tempered” by subsequent passes/layers is significant.

Figure 10
Macrograph of the TB weld with HAZs and weld metals. (A) HAZs in the TB coupon weld; (B) Zones of unaffected weld metals.

3.3.3. Microstructure and hardness profiles

Figure 11A shows the microhardness graph of the welds on AW, PW, and TB coupons obtained in ASME Profile A1. It is possible to notice similarities in the hardness values of the weld metal applied with different welding techniques, which were approximately 285 HV_2/10. The HAZ_HARD in the AW coupon presented very high microhardness values, in the range from 400 to 489 HV_2/10, (average: 426 ±33 HV_2/10). In turn, in the TB coupon, this region (average: 318 ±27 HV_2/10) exhibited a hardness value similar to that in the PW coupon (average: 318 ±17 HV_2/10). Toward the inside of the weld metal (fusion zone), the microhardness of all the regions in the AW, TB, and PW is very similar, as found in the results obtained with Profile A2 (Figure 11B).

Figure 11
Hardness profiles of the AW, TB, and PW test welds. (A) ASME hardness Profile A1 of the AW, TB, and PW coupons; (B) ASME hardness Profile A2 of the AW, TB, and PW coupons; (C) General hardness Profile 3 of the AW, TB, and PW coupons.

Figure 11A shows that the hardness of the HAZ_HARD in the AW coupon surface welds is quite high (between 400 HV_2/10 and 489 HV_2/10) compared to those of the HAZs in the PW and TB coupons. Regarding inner passes (below the cap layer), which were examined in Profile A2 (Figure 11B), the hardness of the weld metal, the HAZ_HARD, and the HAZ_SOFT were quite similar. The behavior of the first pass (Profile 3) is very similar to the inner passes (Profile A2), as shown in Figure 11C.

The hardness of the weld metal is very homogeneous: 258 ±20 HV_2/10 in AW, 250 ±14 HV_2/10 in PW, and 245 ±18 HV_2/10 in TB. These values are lower than the hardness obtained in the Higuchi tests, where it ranged between 429 HV_2/10 and 455 HV_2/10. The HAZ_HARD presented values like those of the original base metal (286–300 HV_2/10), but lower than those found in the Higuchi test, in which hardness was as high as 565–596 HV_2/10. The (over-tempered) HAZ_SOFT exhibited hardness values like those in the Higuchi test, with values between 242 HV_2/10 and 265 HV_2/10, but below the 285 HV_2/10 of the V320 Q&T steel. The HAZ_SOFT experienced peak temperatures above the V320’s tempering temperature, which is around 600°C. There is a gradual recovery of hardness from the HAZ_SOFT to the hardness of the base metal, which produced HAZs around 6–7 mm long.

These results indicate that, in general, the welding applied with the Temper Layer/Bead (TB) technique shows some microstructural homogeneity without excessively hard regions.

The fact that different regions in the weld undergo multiple thermal cycles during welding leads to a complex collection of microstructures that change from one point to the next. To facilitate the microstructural analysis of the TB weld, several representative regions were selected. They were labeled M1 (in Profile 1); M2 (weld metal and HAZ of Bead 5 parallel to Profile A1); M3 (weld metal and HAZ of Bead 1 parallel to Profile A1); and MP1, MP2, and MP3 (in Profiles 2, 3, and 4), as illustrated in Figure 12. To simplify and for reasons of space, only the most significant microstructures in M1 (weld metals) and M2 are shown in this paper.

Figure 12
M1 and M2 regions selected for microstructural analysis.

Figure 13 details the phases or constituents of the weld metal (fusion zone) found in Profile 1 (M1) using the conventions suggested by Linnert [³⁵] and Zhang and Farrar [³⁶]. The microstructures found in the weld metal on the TB coupon had the following constituents which are basically different morphologies of ferrite and other phases [³⁵,³⁶]:

Figure 13
Microstructures in the M1 region of the TB weld metal. (A) Indentations 1–4: GF+AC; (B) Indentations 1–4: GF+AC; (C) Indentations 5–6: PF+AC; (D) Indentations 5–6: PF+AC; (E) Indentations 7 and 9: FS(NA)=NAC; (F) Indentation 10: PF+AC; (G) Indentation 10: PF+AC; (H) Indentations 13 and 14: PF+FC+NAC.

• Indentations 1 to 4. Grain boundary ferrite (GF) —also known as allotriomorphic, proeutectoid, primary, or intergranular ferrite— plus ferrite with aligned secondary phases (AC) that can be martensite, austenite, and carbides (MAC), sometimes referred as Widmanstäten ferrite, which is a ferrite that grows from the grain boundaries into it.

• Indentations 5 to 6. Polygonal ferrite (PF) —also known as ferrite islands— in combination with aligned secondary phases AC (or MAC).

• Indentations 7 to 9. Ferrite with non-aligned secondary phases (NAC or FS(NA)), which is ferrite surrounded by approximately equiaxial microphases and randomly distributed.

• Indentation 10. Polygonal ferrite (PF) and ferrite with aligned secondary phases (AC).

  •Indentations 13 and 14. Polygonal ferrite (PF), ferrite and carbide aggregates (FC), and ferrite with non-aligned secondary phases (NAC).

Figure 14 shows the microstructure of the M2 region obtained in the TB coupon, detailing the weld metal (fusion zone) of Bead B5, the direction of solidification (yellow arrows), the fusion line, and the Heat-Affected Zone (HAZ). Its appearance is quite like those found in the Higuchi test, and the HAZ has the same sub-zones: a HAZ_HARD with high hardness (see Figure 10A), which decreases abruptly to the levels of a HAZ_SOFT with hardness values below those of the V320 Q&T steel (AISI 4140 Q&T).

Figure 14
Microstructures in the M2 region of the TB weld metal (see M2 at Figure 12).

Figure 15 shows the microstructures in the M1 region of the weld metal taken at and near B17 in the AW weld (left: Figures 15A, 15C, and 15E) and at/near B18 the PW weld (right: Figures 15B, 15D, and 15F). Both weld metals (Figures 15A and 15B) contain grain boundary ferrite (GF) and ferrite with aligned secondary phases (AC) that are MAC, also known as Widmanstäten ferrite. Region 1 (R1) of the AW weld (Figure 15C) —which is the HAZ of the B17 on a previous bead— has ferrite AC, but its size is much smaller than that found in other regions of the weld metal or in the TB coupon (Figure 13). Region 2 (R2) of the AW weld (Figure 15E) has a very fine mixture of polygonal ferrite (PF), ferrite and carbide aggregates (FC), and ferrite with non-aligned secondary phases (NAC).

Figure 15
Microstructures in the weld metal on AW (Left) and PW (Right) coupons. (A) Fusion zone of B17 – AW; (B) Fusion zone of B18 – PW; (C) HAZ in R1 of B17 – AW; (D) HAZ in R1 of B18 – PW; (E) HAZ in R2 below B17 – AW; (F) HAZ in R2 below B18 – PW.

Region 1 (R1) of the PW weld —which is the HAZ of B18— (Figure 15D) has polygonal ferrite (PF) and AC with aligned MAC secondary phases, but their size is larger than that obtained in the AW condition. The R2 of the PW weld (Figure 15F) presents a large amount of polygonal ferrite (PF). These microstructural changes produced by the post-welding heat treatment did not have a major effect on the hardness of the molten metals, as is shown in Figure 11.

Figure 16 shows the micrographs of the M2 region of the weld metal (fusion zone), the fusion line, and the HAZs obtained in two cap passes: (A) B18 in the AW coupon and (B) B15 in the PW coupon. The arrows show the solidification path of the weld metal. The HAZ of both beads (B18 in AW and B15 in PW) shows a banded region like that found in the Higuchi tests, with white areas of high hardness values in which the chromium and carbon contents are higher than those in dark areas.

Figure 16
Weld metal and HAZ microstructures in the M2 region of the AW and PW welds. (A) Metallography of B18 in the M2 region of the AW weld; (B) Metallography of B15 in the M2 region of the PW weld.

3.3.4. Mechanical properties

Table 4 reports the results of the all-weld metal tensile tests performed on the AW, PW, and TB coupon welds. The properties of the TB weld were ~20% lower due to the large size of the pores present in this weld (Figure 17). The highest ductility, and highest yield (σ_Y), and tensile strengths (σ_T) were obtained in the PW coupon, and their values were very close to those specified for ER4130. Figure 17 shows the fracture surfaces of the all-weld metal tensile test specimens. The appearance of brittle failure in the TB coupon is in line with its low Reduction of Area (RA), about 10%, and low elongation, approximately 3%. The highest elongation and RA were obtained in the PW coupon (7.7%–8.8% elongation and 22%–26% RA). However, as all the ductility values were very low, bending tests were not performed because they require high bend radio.

Table 4
Mechanical properties determined in tensile tests for AW, PW, and TB coupons.

MECHANICAL PROPERTIES OF TENSION TESTS
COUPON		σT		σY		ELONGATION IN 2” [%]	REDUCTION OF AREA [%]
		[psi]	[MPa]	[psi]	[MPa]
AW	T1	102199	705	89149	615	6.0	16.1
	T2	102014	705	86804	600	7.5	15.0
PW	T1	102851	710	86960	600	8.8	26.1
	T2	104433	720	88896	615	7.7	22.0
CR	T1	79657	550	76635	530	2.5	10.7
	T2	96423	665	87184	600	3.4	9.4

Figure 17
Fracture appearance of all-weld metal specimens (ɸ12.7 mm). (A) Fracture appearance of TB specimen; (B) Fracture appearance of AW specimen; (C) Fracture appearance of PW specimen.

Despite the low ductility and tensile strength values exhibited by the TB coupon sample, it is considered that this is due to the very large pores of this weld metal (see Figure 17A) rather than to a lack of effectiveness of the tempering layer technique. This is confirmed by the hardness results for the HAZ and the weld metal, which were reduced to values like those obtained with the conventional heat treatment of the PW coupon (see Figure 11).

3.5. Summary of the TBW technique

Table 5 reports the hardnesses values obtained in different sub-regions of the TB, AW, and PW coupon welds. This table details the averages and standard deviations of the hardness values in the weld metals, HAZ_HARD, and HAZ_SOFT, considering all the measurements obtained in ASME Profiles A1, A2, and A3. It also includes the maximum and minimum values in each region and the average hardness of the HAZ_HARD in the cap beads (represented by ASME Profile A1).

Table 5
Hardness values (HV_2/10) of three welded coupons: AW, PW, and TB.

	STATISTIC	WELDED COUPON			REMARKS
		AW	PWHT	CR
WELD METAL (WM)	AVERAGE HARDNESS OF WELD METAL	258	250	245	Hardness of WM is similar for all coupons and there are no significant changes associated with the PWHT or the tempering bead technique
	STANDAR DEVIATION OF WM	20	14	18
	MINIMUM HARDNESS OF WM	214	213	214
	MAXIMUM HARDNESS OF WM	297	300	286
HARD HAZ (HAZHARD)	AVERAGE HARDNESS OF HAZHARD	351	315	309	Hardness of AW coupon is ~12% greater than hardness of PW and CR coupons and its dispersion is quite greater than PW and CR
	AVERAGE HARDNESS OF HAZHARD IN A1	426	318	318
	STANDAR DEVIATION OF HAZHARD	64	15	17
	MINIMUM HARDNESS OF HAZHARD	289	286	286	Minimum values are very similar
	MAXIMUM HARDNESS OF HAZHARD	489	338	368	Maximum hardness A1 of AW coupon is 45% greater than PW coupon and 33% greater than CR coupon
SOFT HAZ (HAZSOFT)	AVERAGE HARDNESS OF HAZSOFT	264	260	263	The average hardness was very similar for all the coupons (AW, PW and CR)
	STANDARD DEVIATION OF HAZSOFT	12	12	14
	MINIMUM HARDNESS OF HAZSOFT	242	236	236	Minimum and maximum hardness are very similar in AW, PW y CR
	MAXIMUM HARDNESS OF HAZSOFT	284	281	289

The average hardness of the weld metals in the three coupons was very similar: TB (245 ±18 HV_2/10), AW (258 ±20 HV_2/10), and PW (250 ±14 HV_2/10). Their maximum and minimum values were similar as well (see remarks in Table 5). The HAZ_HARD adjacent to the fusion lines in the TB and PW welds showed a very similar average (309 HV_2/10 vs. 315 HV_2/10), and the minimum hardness was the same (286 HV_2/10). The average hardness of the HAZ_HARD in the AW weld (351 HV_2/10) was almost 12% higher than that of its counterparts in the TB and PW coupons. Hardness peaks up to 489 HV were obtained in Profile A1 of the AW coupon. The maximum and minimum average hardness values of the HAZ_SOFT are very similar in all three welded coupons.

The average hardness of the HAZ_HARD in Profile A1 in the TB coupon is equal to that obtained in its PW counterpart (318 HV_2/10), which is lower than that in the AW coupon (426 HV_2/10). The harder the HAZ, the more susceptible it is to Hydrogen-Induced Cracking (HIC) or phenomena like sulfide stress corrosion cracking [³⁷]. This shows that —compared to the conventional AW pass build-up technique— TBW has a more beneficial effect on the hardnesses of HAZ_HARD in AISI-SAE 4140 steel welded with an ER4130 electrode and, thus, on their microstructures. This can be an advantage for welding repairs when applying a PWHT is not economically, technically, or logistically viable.

4. Conclusions

Regarding the ER4130 filler metal:

• This showed high susceptibility to producing porosity; therefore, it is difficult to use it to fill complete penetration butt joints. A study on this issue is required.

Regarding the welding process and the TBW technique:

• Using GMAW, the TBW technique and an ER4130 filler metal, the hardness values of the HAZ_HARD in cap welds on AISI-SAE 4140 steel tend to resemble those obtained in welds with a post-weld tempering heat treatment (PW). The TBW technique —particularly that recommended in the Section IX of the ASME Boiler and Pressure Vessel Code— can be used in situations where a PWHT cannot be carried out.

• Compared with the conventional AW build-up pass accumulation technique, TBW has a more beneficial effect on the hardness and microstructures of the ER4130 cap weld beads applied to AISI-SAE 4140 steel.

  • The conventional built-up or multi-pass welding technique without a defined sequence —called here “As-Welded” (AW)— produces very hard HAZs below the cap welds (in the range from 400 HV_2/10 to 490 HV_2/10), which are also very brittle and more susceptible to HIC and/or sulfide stress corrosion cracking.

Regarding the mechanical properties:

• It is considered that the low ductility and tensile strength values presented by the TB coupon sample are due to the very large pores of this weld metal and not to the lack of effectiveness of the tempering layer technique. This is confirmed by the hardness results of the HAZ and weld metal, which were reduced to values similar to those obtained with the conventional heat treatment of the PW coupon.

Acknowledgements

The authors would like to thank the Universidad Nacional de Colombia, Sede Medellín, Facultad de Minas, and, especially, the Laboratorio de Soldadura in said campus for their support, financing, and infrastructure, which made this study possible.

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Datasets related to this article will be available upon request to the corresponding author.

• How to cite:
  Morales Galeano JE, Giraldo Barrada JE. Evaluating the effectiveness of the Temper Bead Welding technique in AISI 4140 steel using GMAW with electrode ER4130. Rev. Soldag. Insp. 2025;30:e3011. https://doi.org/10.1590/0104-9224/SI30.11

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• Editor:
  Cleiton Carvalho Silva.

Publication Dates

• Publication in this collection
  31 Oct 2025
• Date of issue
  2025

History

• Received
  01 July 2024
• Accepted
  14 July 2025