GMAW OF CHROME MOLY STEEL TUBES WITH SHIELDING GAS ALTERNATOR

N. RAJASEKARAN
Consultant & Former Dy GM.,
BHEL, Tiruchirappalli, India

A. SANTHAKUMARI
Addl.GM (WRI & Labs),
BHEL, Tiruchirappalli, India

Dr. A. RAJA
Consultant & Former Addl. GM.,
WRI, Tiruchirappalli, India

Abstract

The composition chrome moly alloy steel Tubes make it ideal for use in power plants, refineries, Petro-chemical plants, and oil field services where fluids and gases are transported at extremely high temperatures and pressures.  These tubes offer the advantages like Consistent Workability, Preferred Fabrication and better Mechanical Properties. These tubes are often used in the water wall tubes, reheater tubes and super heater tubes of super critical and ultra-super critical boilers.  Pulsed Gas Metal Arc Welding (P – GMAW) process is widely used for productive joining of these tubes for those applications. In conventional P – GMAW process argon and CO₂ gas mixture is used as shielding gas.  The mixed gas is supplied to the weld pool either as a mixed gas form a cylinder or through a gas mixer. 

Using alternating shielding gases in the weld pool is a method wherein two different shielding gases are alternatively supplied to the torch for effectively protecting the weld pool from the atmospheric contamination. The alternating supply of two different shielding gases to the arc in GMAW produces an effect similar to pulsed current GMAW but dynamically more superior. 

In this experimental work, GMAW with alternating shielding gases of argon and CO₂ has been tried for welding of the chrome moly steel tubes viz. ASTM A 213 T12 and ASTM A 213 T 22 butt joints.

The properties of weldments with alternating shielding gas and conventional supply of shielding gases with mixing units have been evaluated.  The arc experiences switching from spray transfer to short-circuiting within the weld puddle due to alternating shielding gases. This causes stirring in the molten weld pool and positively affects the weld pool thereby minimizing spatter and porosity during welding. The bead characteristics of welds deposited with gas mixer have been compared to the characteristics obtained by welding with alternating shielding gas supply. The alternating supply of shielding gas in GMAW is found to consume less argon thereby provide significant cost savings in shielding gas. 

Key Words: Chrome Moly Steel, Tubes, P- GMAW, Alternating shielding gas, Argon, CO₂

1.0        INTRODUCTION

Gas Metal Arc Welding process is employed widely in almost all industries for enhanced productivity and quality.  For welding of ferrous based materials, a mixture of argon + CO₂ is used as the shielding gas. Shielding gases are being used either as single or as a blend of two or more gases, according to the requirement. In addition to its shielding function, each gas or gas blend have unique physical properties that can have a major effect on penetration, mechanical properties, weld appearance and shape and arc stability.  The current alternatives for the mixed gases is either using premixed gas cylinders or by using a gas mixer in the pre-determined ratio. For structural applications the ratio may be 80% Argon and 20% CO₂. Form high temperature and high pressure applications, it may be in the range of 95:5 or even 98:2. The welding may be carried out in regular spray or pulsed spray mode.

Pulsed spray GMAW process is extensively used to weld tube to form various pressure parts components of a boiler.  The materials used for pressure parts of a boiler is inclusive of stainless steel, chrome  moly steels, low carbon steels and alloy steel tubes and pipes depending upon the pressure and temperature for which they will be subjected to during service.

Chrome Moly tubes are normally cold drawn and manufactured under MIL-T- 6736 B (AMS-T-6736A), AMS-6371 and AMS-6360 standards. These tubes are heat treatable, though hard enable they are easily machined.

Welded areas require heat treatment after welding to retain strength. Chrome Moly tubes are also known as Low Alloy Steel Tubes. General Abbreviations of chrome moly tubes are chrome-moly, cro-moly, CrMo, CRMO, CR-MOLY ASTM A213 are often called as chrome moly tubes because of the chemical contain of Molybdenum (Mo) and Chromium (Cr). Molybdenum increases the strength of steel and Chromium (or chrome) is the essential constituent of stainless steel. The composition chrome moly alloy steel Tubes make it ideal for use in power plants, refineries, petro chemical plants, and oil field services where fluids and gases are transported at extremely high temperatures and pressures.  These tubes offer the advantages like Consistent Workability, Preferred Fabrication and  better Mechanical Properties.

Chrome moly tubes under ASTM & ASME specification A/SA 213 T1, T2, T11, T12, T15, T22, T91, T92, T122 are used for power industries whereas ASTM/ASME A/SA 213 T5, T9 are used for petrochemical industries [1, 2].

For years, Mo has been a standard alloying element used to produce creep-resistant steel capable of withstanding temperatures up to 530 °C. This is because Mo decreases the creep rate of steel successfully, and also slows the coagulation and coarsening of carbides during high-temperature use. Furthermore, this high-temperature suitability and creep-resistance mean the key application of Mo-based steel was in power generation and petrochemical plants.

However, continually increasing the Mo content of the steel in order to further improve its properties does not work since creep ductility actually decreases with increasing Mo. Another limitation refers to the fact that graphitization (breaking down of iron carbides) takes place above 500 °C. These drawbacks hinder the application of Mo-based steels.

A solution was discovered by alloying chromium with molybdenum. This gives the steel a number of advantages not found in Mo-based alloys, and CrMo steels were the first to allow steam temperatures in power stations to exceed 500 °C.

The chrome moly steel tubes are used in water walls, reheaters and super heaters of super critical and ultra-super critical boilers.  Butt joining of these tubes are carried out by pulsed GMAW or GTAW processes to meet the quality requirements as per ASME or equivalent code.

As indicated earlier, the conventional Pulsed GMAW process uses the argon + CO₂ mixture as shielding gas.   The mixed gas is supplied either through a has mixer also known as gas mixing unit or form a pre mixed cylinder or source.

There is another alternative now available for improving the shielding effect by using alternating shielding gas technology by using gas alternator.

In this study, it is proposed to evaluate the effect of alternating shielding gas and gas mixer on the properties of SA 213 T 12 and T 22 tube weldments.

2.0        SHIELDING GAS ALTERNATOR

FIGURE 1

The operating point in GMAW process depends on the shielding gas used; besides the wire feed speed [current] and voltage settings [3, 4].   However, the stable operating point for each shielding gas viz. Argon and CO₂ is located differently on the parametric window. By use of alternating shielding gases wherein two different shielding gases are alternatingly supplied to the torch for effectively protecting the weld pool from the atmospheric contamination.  The arc dynamics changes alternately in tune with the alternating shielding gas supply. The frequently changing arc dynamics positively influences the weld pool thereby the incidence of defects like porosity and crack are decreased. Besides, it also results in improved weld metal mechanical properties in steel.Other factors such as flat bead profile and smooth weld metal transfer are considered to be beneficial aspects of gas pulsing in GMAW process. GMAW with alternating shielding gases is characterised by the switching of the transfer mode from spray to short circuiting type, which produces reliable fusion and penetration. Gas pulsing frequency and procedures have to be established to meet the quality requirements of tube butt joints.Figure 1 shows the Principle of operation of Alternating Shielding gases GMAW.

The typical set up for GMAW process is shown in Figure 2. Figure 2a shows the Conventional GMAW system with Gas Mixer.  The gas mixer delivers the mixed shielding gas to the welding gun / torch.

As shown in Figure 2b, in the gas alternator set up, two different input (Ar& CO₂) goes into the gas alternator and it gives the single output of the set required ratio (0.1 sec to 9 sec). The arc length, structure varies significantly during the switching of shielding gas from argon to carbon dioxide giving the molten weld pool a vigorous stirring effect.

From the schematic shown in Figure 2c, it could be observed that the gas mixing unit is retrofitted with shielding gas alternator in place of gas mixer and no other change is required in the set up.

Figure 2a : GMAW with Gas mixing unit.

 Figure 2b : GMAW with Shielding Gas Alternator.

Figure 2c : Schematic Arrangement for GMAW.

3.0        INFLUENCES ON ARC CHARACTERISTICS

As indicated earlier, periodically alternating the shielding gases between argon and CO₂ significantly influences the arc characteristics. To understand the concept in a better way, the current and voltage signatures are captured using the Analyzer Hannover system, developed by Late Prof. Dr. Ing. Rehfeldt of Germany. The current and voltage transients are captured to study the effect of alternating shielding gas effect on the arc. The current and voltage transient signatures are shown in Figure 3.

Figure 3 : Current and voltage transients in GMAW under alternating shielding gases of argon (0.8 s) & CO₂ (0.2 s).

During the study, the argon flow was maintained for 0.8 sec and CO₂ for 0.2 sec and the cycle has been repeated. The smooth horizontal line segments in the current, voltage transient signature indicate the argon shielding phase where the arc is smooth with spray mode of transfer and the segments with spikes in the current, voltage transient signature indicate the CO₂ shielding phases where arc changes to short circuiting mode. Nearly 8 to 10 short circuits occur within the span of 0.2 seconds. This superimposition of short duration short circuit transfers of CO₂ in the regular spray transfer of argon creates a stirring effect on the weld pool which positively influences the process. This enables to reduce the porosity level significantly and results in better microstructure of the weld.

4.0        EXPERIMENTAL DETAILS

GMAW with alternating shielding gas is a new technology. The integrity of this process for welding of chrome moly steels has been evaluated through experiments.

Trials have been done at tube to tube joining machines on ASTM SA 213 T 12 and T 22 materials which are of conventionally used for high temperature high pressure applications. The following are the sample tube that have been taken up for study and the dimensions are given below.

• ASTM SA213 T12 -Ø57.15mm*14.3mm
• ASTM SA213 T22 -Ø51mm*5.6mm

The materials have been edge prepared by the facility available and welding trials have been done with the conventional process of welding with gas mixing unit on the chosen material and with the alternator developed by WRI. The parameters which have used for the welding trials are tabulated in Tables 1&2.

Number of experiments done prior to set the frequency while using alternator and the optimised parameters which have been used for welding trials are furnished in the following table.

Preheating has been done as per requirement using LP gas before welding.  After welding, the samples are subjected to PWHT also as per ASME code requirement.

Transverse tensile and impact specimens have been extracted from the welded tubes and tested as per AWS standard. Microstructural studies have been observed in the base metal and weld metal in different shielding gas conditions.

Table 1: Parameters for GMAW with Gas Mixer.

Parameter	SA 213 T 12	SA 213 T 22
No of passes	5	2
Welding current (A)	95, 94, 93, 92, 92, 93	93, 92, 90
Welding voltage (V)	24, 24, 23, 22.5, 22, 22.5	23.5, 23, 22
Weld speed (mm / min)	240, 120 ,100 ,80, 80, 90	320, 200, 210
Oscillation speed (mm / min)	30, 2400, 2100, 1800, 1800, 1800	40, 2600, 2700
Shielding gas Ar + CO₂%	95% + 5%	95% + 5%

Table 2: Parameters for GMAW with Gas Alternator.

Parameter	SA 213 T 12	SA 213 T 22
No of passes	5	2
Welding current (A)	94, 95, 95, 94, 96, 95	94, 93, 92
Welding voltage (V)	25, 24, 24, 24, 25, 22.5	24, 24, 22
Weld speed (mm / min)	270, 120, 80, 85, 80, 90	330, 200, 210
Oscillation speed (mm / min)	30, 2000, 1800, 1600, 1500, 1600	40, 2600, 2700
Shielding gas Ar + CO₂%	Ar – 0.02 sec + CO₂ – 0.04 Sec	Ar – 0.02 s + CO₂ – 0.04 s

5.0        RESULTS AND DISCUSSIONS

The mechanical properties of welds deposited by GMAW with alternating supply of argon and CO₂ shielding gases have been studied and compared with that of the weld metal deposited by GMAW with gas mixers.

5.1        NDT-Real Time Radiography (RTR) Test

The two variants of Chrome Moly steels (T12 and T22) were welded were subjected to Radiography (RT) test to ensure the soundness of the joint.  All the joints met the RT requirement as per ASME.

5.2        Tensile Test

The tensile strength of the weld was determined by using UTM 600 kN. The transverse tensile test specimen is prepared as per the standard AWS B4.0.

Table 3 shows the transverse tensile test results for SA 213 T12 tubes and Figure 4 shows the specimens after testing.

Table 3: Tensile Results for SA 213 T12 Tubes.

ID No.	GAS MIXER
	UTS (MPa)	Location of failure
T12-C31	467	Base Metal
T12-C32	470	Base Metal
	GAS ALTERNATOR
ID No.	UTS (MPa)	Location of failure
A11	473	Base Metal
A44	463	Base Metal

From Figure 4, it can be seen that the fracture occurred in the base metal of both weld methods. So, the weld metal has better tensile strength than the base metal and meet the code requirements.

Figure 4 : SA 213 T12 Tensile Specimens After Testing.

Table 4 shows the transverse tensile test results for SA 213 T22 tubes and Figure 5 shows the specimens after testing.

Table 4: Tensile Results for SA 213 T22 Tubes.

Figure5: SA 213 T22Tensile Specimens After Testing.

 From Figure 5, it can be seen that the fracture occurred in the T22base metal of both weld methods. So, the weld metal has better tensile strength than the base metal and meet the code requirements.

5.3        Bend Test

The welds were subjected to guided bend tests and test coupons were prepared as per AWS B 4.0.  Both transverse face and root bend tests were carried out for T22 tubes and side bend test for T12 tubes, to evaluate both the ductility and soundness of the weldments.  180° bend tests with mandrel diameter equal to 4t were carried out on all welded samples.  Both the specifications were tested and the results are tabulated in Tables 5& 6.  The face and root bend tested images are shown from Figures 6 and 7.

GAS MIXER
ID No.	Side Bend	Remarks
C41	3.25 mm Open discontinuity	Failed
GAS ALTERNATOR
A12	No open discontinuity	Passed

Figure 6 : SA 213 T12 Specimens After Face & Root Bend Testing.

Figure  7 : SA 213 T22 Specimens After Face & Root Bend Testing.

GAS MIXER
ID No.	Face Bend	Remarks	ID No.	Root Bend	Remarks
T22-C41	No open discontinuity	Passed	T22-C42	No open discontinuity	Passed
GAS ALTERNATOR
A44	No open discontinuity	Passed	A45	No open discontinuity	Passed

5.4        Charpy V – Notch Impact Test

To compare the toughness properties of both the weldments from T22 samples were subjected to Charpy V notch impact test. The impact test specimens were prepared as per the standard AWS B 4.0.  The test results are tabulated in Table 7 and Figure 8 shows the tested samples.

The average value for gas mixer is 70.67 J and for gas alternator is 96 J.  From the test values, it is evident that the weld metal deposited with alternating shielding gases GMAW has better toughness properties as compared to the weld metal deposited with gas mixer.

Figure 8: Impact Specimens for SA 213 T22 Material after Testing.

Table 7: Impact Results for SA 213 T22 Material.

GAS MIXER		GAS ALTERNATOR
ID No.	Impact Energy in J	ID No	Impact Energy in J
C1	72	A1	90
C2	81	A2	104
C3	44	A3	106
C4	85	A4	77
C5	59	A5	94
Avg.	70.67		96

5.5        Heat Input

The heat input per pass was calculated forboth the materials based on the welding voltage, welding current and welding speed and compared for both types of welds made with gas mixer and gas alternator.

From Table 8, it can be seen that the heat input for the welds made with gas alternator is significantly lesser than that of welded with gas mixer.  This also can be attributed for better mechanical properties and resulted in fine grain size.

Table 8: Heat Input.

Spec.	Volt	Amp	Speed	Heat input rate kJ / mm
				Mixer	Alternator
T12	23.91	93.5	115	1.167	—
	24.08	94.83	120.83	—	1.134
T22	23.33	93	246.66	0.527	—
	22.83	91.67	243.33	—	0.516

5.6        Effect on Heat Affected Zone (HAZ)

From Figure 9 it could be observed that the width of HAZ is 5.29 mm is for the weld with gas alternator against 5.56 mm for SA 213 T 12 gas mixer welds and this may be due to the low heat input in alternating shielding gas supply to the weld pool.  For SA 213 T22, it is 6.407 mm for gas mixer and 5.36 mm for gas alternator for the same reasons, as shown in Figure 10.

The improved tensile and toughness properties of the weld metal deposited with alternating shielding gases of argon and CO­₂ as compared to mixing unit is attributed to the fine grain size and improved microstructure. Grain size has a strong effect on transition temperature. An increase of one ASTM number in the ferrite grain size (a decrease in grain diameter) can result in a decrease in transition temperature [5]. Decreasing the grain diameter decrease the transition temperature and improve the impact resistance.

5.7        Microstructure

The microstructural study was done under the optical microscope under proper illumination condition in the desired region, under 500X magnification.

The microstructure of base materials, HAZ and weld metal for SA 213 T12 tube are shown in Figures 11 to 13 respectively.

Figure 11: SA 213 T12 Base Metal Microstructure 500X.

 Figure 12 : Microstructures of SA 213  T12 with gas mixer and gas alternator HAZ.

Equal amount of polygonal ferrite and bainite was seen in base metal. Hard martensitic phase was observed at the HAZ; also prior austenite grain boundary is seen (Figure 12).

From Figure 13, it can be seen that for SA 213 T12, the bainite structure is formed at the weld metal region in both gas mixer and gas alternator The coarse polygonal ferrite was noticed together with bainitic phase at middle and bottom of the weld metal of gas mixer when compared to gas alternator.

The microstructures of the base material, HAZ and weld metal for SA 213 T22 tubes in Figure 14 to Figure 16, respectively.

Figure 14: SA 213 T22 Base metal microstructure 500X.

Figure 15S: A 213 T22 gas mixer and gas alternator HAZ microstructures 500X.

The HAZ microstructure of T22 steel Gas Mixerand Gas Alternator in shown in Figure 15. Prior Austenite Grain Size (PAGS)was finer in gas alternator compared to gas mixer.

The weld metal microstructure of T22 steel Gas Mixer and Gas Alternator in shown in Figure 16. Bainitic structure was present in the weld metal.In gas mixer the grain or size of bainite formed in the weld metal are coarser. This coarser grain structure result in lesser hardness