Effect of Mo and Ni addition on Mechanical and Microstructural properties of Normalized and Stress relieved Shielded metal arc welded joints

Abstract: Low alloyed steel consumables are widely used for material joining applications because of their adequate mechanical properties and cost. While selecting a specific electrode or filler for joining operation chemistry and mechanical properties play an important role. For some critical applications there are heat treatment requirements to enhance mechanical properties and grain refinement such as normalizing, stress relieving etc. Each and every alloying element has individual and combined mechanical behaviour with other elements. Weld chemistry and heat treatments make positive or negative impacts on mechanical properties. For this study three types of low alloy steel electrodes like C-Mn, C-Mn-Mo, & C-Mn-Ni were used to make welded joints with Shielded metal arc welding process. The properties of undiluted weld metal of above three types of electrodes are evaluated under condition of as welded, stress relieved and normalising followed by stress relieved. This paper reports on tensile strength, impact toughness properties, hardness and microstructure of above-mentioned electrodes undiluted weld metal. The undiluted weld metal microstructure and fracture morphology were examined under optical and scanning electron microscope. Effect of alloying elements on ductile to brittle transition temperature and phase analysis through X-ray diffraction was discussed.

Keywords: Shielded metal arc welding, Heat treatment, Mechanical properties, Ductile to brittle transition temperature, X-ray diffraction.

1. Introduction

At present, there is a strong demand in the steel industry regarding development of high strength micro alloyed steels for satisfying ever increasing industrial needs, such as pipeline, automotive, and bridge building.[1] Molybdenum, Niobium, vanadium are commonly used elements. They have large atomic affinity with carbon, and can greatly form carbides with high hardness, high melting point and good stability. [2] These alloying elements are easier to incorporate for welding operation with respect to welding electrodes which are used in Shielded metal arc welding (SMAW) which is a manual operation in which an arc, creating a pool of metal, is formed among the shielded electrode and work piece.[3] Adjusting the content of alloying elements can change the C partition in carbides and substrate, Controlling the microstructure and mechanical properties of steels. Multiple heat treatment processes are often used for grain refinement and obtaining proper microstructure and mechanical properties [4].

The effects of post-weld stress relieving and normalizing followed by stress relieving processes in Mo an Ni addition on the microstructure, tensile properties and impact toughness of C–Mn–Si steel weld metal were investigated systematically. The research provides guidance for the design of post-weld heat treatment of steel and property controlling of weld metal. Also, it is focused on variation of alloying elements to optimum level to get optimum mechanical properties such as tensile, impact and hardness of all weld metal.

2. Experimental Procedure 

CMn, CMnMo and CMnNi electrodes of 4x450mm size

Table 2.1 Chemical composition.

were selected to make welded joints with C-Mn steel (SA 516 Gr.70) as base material using SMAW process. Table 2.1 shows the actual chemical composition used for the study by making chemical pads and analysed through optical emission spectrometer. Dimensions for weld joint, welding parameters like root gap, Bevel angle, Interpass temperature, Back plate and Base plate dimensions were selected as per the ASME section II C – SFA 5.5. Three Welded joints each for chemical composition specified in the table 2.1 were prepared and then subjected to the following conditions for evaluation of their mechanical and microstructural properties.

• As Welded.

• Stress relieving at 620° C.

• Normalizing 920° C followed by Stress relieving at 620° C

The base material used in the present investigation was SA 516 Gr. 70 plates basically low carbon steel of sizes 450mm X 125mm X 20mm with Backing plate of size 470mm X 30mm X 10mm. All the joints are welded in (1G) position and DCEP polarity was used. The welding parameters used for all the welded joints are 160-165 Amps and 22-23V with average heat input of 1.5-1.9KJ/mm. The dimensions of welded joints and orientation of specimens for mechanical testing are as per fig 2.1 and 2.2. The dimensions for impact and tensile samples are with respect to ASTM E23 and ASTM E8 respectively [].

Fig: 2.1 Dimensions of Welded joints.

Fig: 2.2 Orientation of impact and tensile sample.

Fig 2.3: Assembly after Welding.

Charpy V Notch tester (FIE make) and Universal Testing machine (UTM) of 400 KN capacity was used for testing of samples. Brinell hardness of all weld metal was measured according to ASTM E10.

3. Results and Discussion

3.1 Tensile and Yield strength of weld metals

The tensile specimens were obtained from the centre of the welded plate which possess all weld metal as the composition. The results from Table 3.1 shows that Variation 0.4% Mn has no appreciable effect on Ultimate tensile strength and Yield

Table 3.1 Effect of heat treatment on UTS and YS.

Table 3.2 Effect of heat treatment on % Elongation.

Table 3.3 Effect of heat treatment on Brinell hardness.

strength if as weld condition in considered. Stress Relieving reduces the UTS and YS by 30 to 40 Mpa, however normalizing followed by stress relieving reduces UTS by 100 to 110 Mpa and YS by 160 to 180 Mpa.

Addition of Mo combined with Mn in weld metal results in overall enhancement of UTS and YS. Variation of 0.2% Mo results in increase in UTS and YS about 30 Mpa. Stress Relieving has no appreciable effect on strength as compared to as welded condition. Normalizing followed by stress relieving tends to decrease the strength of the weld metal to a greater extent about 150 to 200 Mpa as that of the as welded state. Addition of Ni combined with Mn in weld metal also leads to enhancement in strength of the weld metal. Variation of 0.4% Ni results in increase in the strength by 25 to 30 Mpa. Ni after stress relieving tends to decrease the strength by 30 to 35 Mpa and after normalizing followed by stress relieving decrease in strength of about 100 to 120 Mpa was observed as compared to as welded condition.

3.2 Hardness values of weld metals

It can be observed from the table 3.3 that as the heat treatment progresses from AW state to Stress relieving to normalizing the resistance of material to deformation decreases indicating a fall in values of BHN except for 0.42 % Mo trials which gives higher hardness after stress relieving.

Mn addition has no considerable effect on hardness of the weld metal however in Ni addition such as 0.8% Ni gives better results when combined with Mn. Overall effect of heat

Table 3.1 Effect of heat treatment on UTS and YS.

treatment on the hardness values are not beneficial as it makes material softer which results in decreased values of hardness.

3.3 Impact strength of weld metals

From table 3.4 it is observed that both as welded Mn trials can last up to -66 degree Celsius. Mn on stress relieving increases the impact toughness by 20 to 30% and lowers the ductile to brittle transition temperature. Mn on normalizing followed by stress relieving drastically reduces impact toughness and increase DBTT. Addition of Mo has a negative effect on the impact toughness in terms of other alloying elements Mo reduces the impact toughness and increases the DBTT. Mo after normalizing followed by stress relieving drastically reduces impact toughness and increase DBTT.

Addition of Ni in weld metal drastically increases the impact toughness. Ni on stress relieving shows decrease in elongation but increase in impact toughness and lowers DBTT. Ni on normalizing followed by stress relieving results in increasing the DBTT and lowering energies of impact.

Fig: 3.1 As Welded 0.4% Ni Top bead 200x.

Fig: 3.2 As Welded 0.4% Mo Top bead 500x.

3.4 Microstructure analysis

From each heat-treated condition some samples are picked and optical microscopy after 5% Nital etching is carried out of top bead in the weld metal. The as welded microstructure shown in Fig 3.1 and 3.2 has different ferrite morphologies such as acicular ferrite, side plate ferrite and M-A-B constituent. As the process of welding is very fast and inhomogeneous, pearlite (alternate layers of ferrite and cementite) is not seen in the as welded condition. The structure obtained is more needles like and basket weaved.

Fig: 3.3 Stress relieved 0.8% Ni Top bead 500x.

Fig: 3.4 Stress relieved 0.4% Ni Top bead 500x.

Fig 3.3 and 3.4 shows no phase change after stress relieving, randomized arrangement of ferrite morphologies with some of the polygonal ferrite arrangement associated with side plate and acicular ferrite structure is seen.

It is observed that the original as welded microstructure after normalizing followed by stress relieving has converted into a globular coarse grained ferrite structure with pearlite

Fig: 3.5 Normalized and stress relieved 1.57% Mn Top 500x.

Fig: 3.6 Normalized and stress relieved 0.8% Ni Top 200x.

associated at the grain boundaries also it could have removed the residual stresses caused by welding [5,6]. Scanning electron microscopy and Energy Dispersive Spectroscopy of a sample which has shown higher value of hardness has been carried out as shown in Fig 3.7 and 3.8, from the EDS it is clarified that the microstructure consists of ferrite morphologies and considerable amount of MoC are present.

Fig: 3.7 As Welded 0.4% Mo Top 5000x.

Fig: 3.8 EDS Analysis of AW 0.4% Mo.

3.5 Inclusions Analysis

The surface morphology of the fractured impact samples was examined along with the inclusions present in them using a Scanning electron microscope. EDS analysis of inclusions shows the type (oxide, sulphide) and their morphology. Fig 3.9 shown below clarifies about the inclusions present in the structure.

Fig: 3.9 Oxide Inclusions present on fractured surface of 0.8%Ni normalized impact sample.

Table: 3.5 EDS Analysis 0.8%Ni normalized impact sample.

The sample which has been tested for inclusion analysis contains 0.8% Ni normalized followed by stress relieving and shows high impact toughness. The elements present mainly show the presence of oxygen and Titanium. From this the samples which are having higher values of impact toughness show large number of oxide inclusions on their fracture morphology.

Similarly, the Fig shown below contains 0.8% Ni in Stress relieved condition also shows presence of number of oxide inclusions on their fracture morphology.

Fig: 3.10 Inclusions present on fractured surface of 0.8%Ni Stress relieved impact sample.

Table: 3.6 EDS Analysis 0.8%Ni stress relieved impact sample.

4. Conclusions

From this study, following conclusions are derived.

• Addition of Mo and Ni in weld metal increases the hardness. Mo addition had a better result among all the other trials in terms of hardness as increase in the hardness up to 25 BHN is observed as compared to non-addition.

• Addition of Mo in weld metal increases the YS by 80-100 Mpa and UTS by 120-150 Mpa. Mo with stress relieving and as weld condition has similar results in terms of strength but reduction in impact toughness and increase in the DBTT above -45 0C is seen.

• Addition of Ni resulted in increase in YS by 20 to 25 Mpa UTS by 30 to 40 Mpa. Ni on Normalizing and stress relieving results in lowering the DBTT below -66 0C.

• Normalizing combined with stress relieving has shown to promote polygonal and globular ferritic structure with pearlite.

• Oxide inclusions present in Ni trials have promoted acicular ferrite formation which resulted into better impact toughness.

References

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