Figure 1. Microstructure of as-received super duplex stainless steel SAF 2507.

Effect of rotational speed on intergranular corrosion resistance of friction welded SDSS SAF 2507 joints

In this research paper super duplex stainless steel SAF 2507 has been selected to investigate the intergranular corrosion resistance of friction welded SDSS SAF 2507 joints. The joints were welded at three different rotational speeds: 700 rpm, 1080 rpm and 1600 rpm and three different friction pressures: 40 Mpa, 69 Mpa and 78 Mpa.
 
Intergranular corrosion resistance was detected using the Huey test for three periods (48, 96 and 144 hours) and average values taken. The joints were also subjected to metallographic study. The results showed that increasing friction pressure and rotational speed increases the intergranular corrosion rate. Maximum intergranular corrosion rate occurs at specimens welded with the highest rotational speed of 1600 rpm and 78 Mpa friction pressure. The results attributed to changes in microstructure in fully plastically deformed zone FPDZ and partially deformed zone as a result of heat and pressure.

Article by Assistant Prof. Ramadhan H. Gardi, Prof. Sherko A. Baban and Ahmed A. Ahmed, MSc. student at College of Engineering, Salhaddin University, Iraq
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Introduction

The austenitic-ferritic microstructure of duplex stainless steel combines the attractive properties of austenitic-ferritic stainless steel. Super duplex stainless steel SAF2507 is used in the oil and gas industry, as tubing for heat exchangers in refineries, pipes for seawater transport, propeller shafts and other products subjected to high mechanical loads in seawater and other chloride containing environments[1].

Friction welding involves bonding between a stationary and rotating member by the frictional heat generated while subjected to high normal force on the interface[2,3].The frictional heat generated at the interface rapidly raises the temperature of work pieces to values approaching the melting range over a short axial distance[4].

Sathiya et al[5] studied the friction welding of AISI 304 austenitic stainless steel. Sahin and Akata[6] directed an experimental study on friction welding of medium carbon steel and austenitic stainless steel. Ochi et al[7] also studied tensile strength of SUS 304 stainless steel friction welding joints. Satyanarayanan et al[8] studied the mechanical properties of friction welded austenitic –ferritic stainless steel dissimilar joints P. Sathya et al[9] recommended friction welding parameters to achieve better properties of ferritic stainless steel weldment. 

G Madhusudan Reddy and K. Srinavansan Rao[10] studied notch tensile strength and impact toughness of friction welded duplex stainless steel SAF2205 and compared to tensile strength and impact toughness of electron beam welded of the same material, they found that the tensile strength of the friction welded specimen was lower than that of electron beam welded. A Ravi Shankar et al[11] studied the effect of friction welded parameters on the intergranular corrosion rate of dissimilar friction welded joints between zircaloy -4 and 304L stainless steel using ASTM A 262 practice- C and they concluded that all the corrosion occurred on the 304 L stainless steel and at the joint interface.

Literature available on corrosion resistance of friction welding of super duplex stainless steel joints is very scarce. In the present study super duplex stainless steel SAF2507 joints were processed by various friction welding parameters. The joints were subjected to metallographic study and the influence of friction welding parameters on intergranular corrosion of friction welded joints was detected.


Experimental procedure

Material: The material used in this study was a 20 mm diameter round bar of Sandvik SAF 2507 super duplex stainless steel [UNS 32750]. The composition of parent materials given in Table 1. and Figure 1. show the microstructure of super duplex stainless steel SAF 2507 in as-received condition.

Friction welding

Friction welding was conducted using a continuous drive friction welding machine at three different rotational speeds: 700, 1080, and 1600 rpm. The friction pressure was varied in two different cases (40 Mpa and 78 Mpa) whereas the forging pressure and friction time were constant throughout the experiment and are 86 Mpa and 60 seconds respectively. In the continuous drive friction welding process a stationary member was pressed against a rotating member with axial pressure.

The relative motion between stationary and rotating member produced frictional heat which caused the material to soften and be plastically deformed. After preset displacement the machine was rapidly stopped and the pressure (forging pressure) was increased to generate a high solid state weld. The axial length of the sample was measured before and after welding. The maximum temperature at the joint was calculated.

Metallographic analysis

The weld sample was subjected to standard metallographic preparation to examine the microstructure. The specimens were first mechanically polished with emery paper and then etched according to ASTM standards[12] by ferric chloride and nitric acid reagent which consists of a saturated solution of FeC13 in HCl, to which a little HNO3 is added and etching time equals three minutes. Microstructure of weld interface and heat affected zone were recorded.

Intergranular corrosion test

Nitric acid solution 65 % was used to detect susceptibility to intergranular corrosion according to ASTM. The solution was prepared by adding 108m1 of distilled water per liter of concentrated nitrc acid (72% concentration).

The conical flasks were equipped with 750 ml of the acid solution. Reflux condensers were used to avoid evaporation of the acid solution. The flasks were mounted on the hot plate of an electrically regulated heater. Cooling water was used continuously during the test through the condenser and the acid solution temperature rose to boiling temperature (121°C).

Figure 2 shows the equipment described. Three test cycles were selected to detect the susceptibility of joints to intergranular corrosion and each cycle equalled 48 hours i.e. the total time for all the test cycle equalled 144 hours. After each test cycle the specimens were drawn out from the flasks, rinsed with water and soap, and treated by scrubbing with nylon brush, then dried with hot air and weighed by electrical scale.

The weight loss of the specimens after each test period was measured. The corrosion rate as inches per month was calculated as follows:

Cm per hr ---- Δw/(A x d x t)
Inches per month = (287 x Δ w) / (A x d x t)
Where: t = time of exposure, hr.,
A = total surface area, mm2, Δw = weight
loss, g and, d = density, g/mm3.

Inches per month were converted to the most commonly used unit for corrosion rate (mpy) as follows:
inches per  month × 12000 = mils per year (mpy)


Results and discussion


Table 1. shows the intergranular corrosion rate using the Huey test for three periods (48,96 and 144 hours) and the average values of these for asreceived super duplex stainless steel SAf 2507 friction welded joints. The welded joints were obtained in three different rotational speeds (700 rpm, 1080 rpm, 1600 rpm), different friction pressure and 86 Mpa forging pressure, friction time and forging time respectively.

Table 2 and Figure 3 show that the amount of intergranular corrosion of average value of three different stages in as-received SDSS SAF 2507 is 55.65 mpy.

Figure 3 shows that by increasing rotational speed from 700 rpm to 1080 rpm the amount of intergranular corrosion rate in SDSS SAF 2507 joints increased for the same friction welding parameters 78 Mpa friction force and 86 Mpa. Increasing the rotational pressure further to 1600, increased the amount of intergranular corrosion to its maximum value 191.6 mpy for the same friction welding parameters.

The figure shows that increasing the friction pressure from 40 Mpa to 78 Mpa when rotational speed is 1080 rpm increases the intergranular corrosion rate from 82.86 mpy to 115.96 mpy. The same phenomenon can be seen at 1600 rpm rotational speed when friction force increased from 69 Mpa to 78 Mpa, which increased the intergranular corrosion rate from 71.25 mpy to 191.6 mpy. This is because the rise in friction force increases the amount of heat at the joints and work hardening in heat affected zone.

* See below for Figures 4 to 9

This leads to the ferrite and austenite phase becoming more elongated, increasing susceptibility to intergranular corrosion. Figures 4, 6 and 8 show the microstructure of fully plastically deformed zone of specimens welded with 78 Mpa friction pressure and 700, 1080 and 1600 rpm rotational speed respectively. As the rotational speed increased the interface reached a high temperature within a short time and led to a lower cooling rate and wider heat affected zone and finally lower grain size.

Figures 5, 7 and 9 show the microstructure of partially deformed zone PDZ of specimens welded with friction pressure 78 Mpa and 700 rpm, 1080 rpm and 1600 rpm respectively. The figures show that as the rotational speed increased the area adjacent to the fully plastically deformed zone which is a partially deformed zone increased and more grain refinement will occur as a result of heat and pressure.

Conclusions

1. Increasing friction pressure at any rotational speed in SDSS SAF 2507 friction welded joints increases the susceptibility to intergranular corrosion.

2. The highest intergranular corrosion resistance can be seen in friction welded SDSS SAF 2507 joints welded with1600 rpm rotational speed and low friction pressure.

3. Increasing rotational speed increases grain refinement at fully plastically deformed zone FPDZ in friction welded SDSS SAF 2507 joints. References available upon request.

About the author

Ramadhan H Gardi is Assistant Professor in the Mechanical department of the College of Engineering at Salahaddin University in Iraq. He has a BSc in Mechanical Engineering and an MSc in Corrosion Engineering, and has presented papers at both Duplex World and Stainless Steel World conferences, as well as publishing papers in several international journals.

Figures 4 to 9:
 

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