OPTIMIZATION OF TIG WELDING PARAMETERS IN FERRITIC STAINLESS STEEL (AISI 430)

(1)

3^ème Conférence Internationale sur

le Soudage, le CND et l’Industrie des Matériaux et Alliages (IC-WNDT-MI’12) Oran du 26 au 28 Novembre 2012, http://www.csc.dz/ic-wndt-mi12/index.php 66

OPTIMIZATION OF TIG WELDING PARAMETERS IN FERRITIC STAINLESS STEEL (AISI 430)

N. Bensaid ¹, N. Tala-Ighil ¹, R. Badji ¹, M. Hadji ²

1: Centre nationale de recherche scientifique en soudage et contrôle (CSC), PB 64, Chéraga Alger, Algérie

2: Département mécanique, Université Saad Dahlab de Blida. BP 270, 09000 Blida, Algérie

Abstract:

Ferritic stainless steels, in general, and the first generation of the group 430, in particular, are associated with many problems during the welding process. These problems are the grain growth, both in the fusion zone (FZ) and heat affected zone (HAZ) and martensite formation at grain boundaries of the weld. The grain growth of ferrite at high temperature and the presence of martensite are that resilience at ambient temperature is generally low, so that the assembly becomes susceptible to fragile fracture. The aim of the work is, first to characterize the microstructure of welds steel (AISI 430) as variable parameters and then optimize its own parameters. We focused on improving refining the grain size of ferritic stainless steel (AISI 430) during the TIG welding (Tungsten Inert Gas). The results obtained in the present study show that, the microstructural characteristics of the weld are influenced by the current and the welding speed and beyond a critical value of current, speed has no influence.

Key words: welding current, welding speed, TIG welding, ferritic stainless steel, Grain Refinement.

1 Introduction

Ferritic stainless steel (FSS) is characterized by lower cost, higher thermal conductivity, smaller linear expansion and better resistance to chloride stress corrosion cracking, atmospheric corrosion and oxidation compared to austenitic stainless steels [1,2].

The principal weldability issue with the FSS is maintaining adequate toughness and ductility in the weld zone (WZ) and heat affected zone (HAZ) of these steels, this is due to large grain size in the fusion zone (FZ) [3,4] because they solidify directly from the liquid to the ferrite phase without any intermediate phase transformation. Tag

The microstructural features of weld metals that influence their after-weld properties are controlled by the energy input and heat transfer factors, which are governed by the welding current, travel speed and the material. The microstructures and grain morphologies determine the strength of the weldment. However, the influence of welding variables such as heat input, of grain modifier on the microstructure and welding

(2)

speed and presence properties of fusion welded ferritic stainless steel has hardly been studied. These variables are critical factors in controlling weld induced microstructure and properties in weldments. [5]

In precise, in this paper we have focused on the improvement in grain refinement of ferritic stainless steel using Tungsten Inert Gas welding.

2 Experimental procedure

The base material employed in this study is 1.2 mm thick AISI 430 FSS. The chemical composition of the base material is presented in Table 1.

Tab.1 Chemical composition of AISI 430 FSS.

C Mn Si Cr Ni P S Fe

wt (%) 0.055 0.42 0.45 17 - 0.03 0.1 Rst

The advantages of TIG welding is a low heat input, less distortion, resistance to hot cracking and better control of FZ by improved mechanical properties. From Literature [6, 7 and 8], the important parameters of the process that have more influence on the geometry of the weld bead and on the refinement of grain size of FZ have been identified [9], they are the welding current, welding speed and arc voltage. These parameters are related by one parameter, which is the rate of heat input per unit length of the weld Easterling [10]. The relationship is:

η is the process efficiency, I is the arc current (A), V is the voltage (V) and v is the welding speed (mm/s).

The process efficiency (η) for the GTA welding process is roughly 48% [11].

The welding conditions and process parameters used to fabricate the joints are given in Table 2.

Tab.2 Welding conditions and process parameters

Parameter Value

Welding current (A) 30-70

Welding speed (mm/s) 0.77- 4.60

Electrode polarity DCSP

Arc voltage (V) 10-14

Electrode 2% Thoriated Tungsten (mm) 1.6

(3)

Shielding gas (Argon), flow rate(L/min) 8-12

The heat input per unit length of the weld is calculated by the formule1. A procedure was followed in order to determine the grain size in the weld bead. The specimens are cut to the cross section of the weld and prepared for the optical metallographic. The grain size is determined by the use of a image analysis software (ImageJ). The results are summarized in Table 3.

3 Results and discussion

The topography of the weld bead specimen produced by varying welding parameters are shown in Fig.1.

Fig.1:Topographie of the weld specimen « S8 ».

The table 3 shows a total of 28 specimens of welds realized for different combinations of current input and welding speed.

Welding current (A)

Welding speed (mm/s)

Heat Input rate/unit length of weld (J/mm)

Grain size (µm)

1 ³⁰ ^1,20 ^167,81 ^255,12

2 30 0,95 151,67 218,56

3 ³⁰ ^0,77 ^219,95 ^247,14

4 35 1,67 130,78 268,11

5 ³⁵ ^1,89 ^106,73 ^202,96

6 35 1,37 172,22 257,68

7 ³⁵ ^1,06 ^205,55 ^260,15

(4)

8 ³⁵ ^1,25 ^187,66 ^241,79

9 35 0,96 228,39 299,96

10 ⁴⁰ ^2,26 ^85,09 ^191,82

11 40 1,32 145,23 232,96

12 ⁴⁰ ^2,02 ^126,45 ^271,06

13 40 1,84 138,71 272,34

14 ⁴⁰ ^1,28 ^199,36 ^251,34

15 40 1,18 217,08 268,24

16 ⁴⁵ ^4,40 ^54,06 ^185,13

17 45 2,49 86,90 192,93

18 ⁴⁵ ^2,17 ^109,66 ^207,74

19 45 2,38 99,90 223,66

20 ⁴⁵ ^1,83 ^118,29 ^225,51

21 ⁴⁵ ^1,60 ^134,68 ^234,79

22 ⁵⁰ ^2,46 ^97,39 ^203,47

23 50 2,53 94,88 209,79

24 50 2,01 119,27 212,78

25 ⁵⁵ ^3,10 ^85,16 ^183,26

26 ⁶⁰ ^3,29 ^87,49 ^195,46

27 ⁶⁵ ^3,46 ^90,17 ^211,64

28 70 4,60 73,043 206,69

Fig 2 shows the microstructure of the 28 welds specimens observed under optical microscope.

(5)

Fig.2: Microstructure of the 28 welds realized.

The microstructural characterization of the welds realized with varying of heat input rate (fig.2), shows the presence of interdendritic martensite in the FZ (Fusion Zone) and boundary grain martensite with carbide presence in the HAS (Heat Affected Zone).

(6)

Fig.3: Evolution of Grain size with the Heat Input Rate.

Figure 3 shows that the grain size increases with the heat input Rate. The grain growth is minimized after welding by reducing the heat input.

Fig.4: Evolution of Grain size with the Welding Speed and Heat Input Rate.

50 75 100125

150175

200 225 250 160

180 200 220 240 260 280 300 320

0,51,01,52,02,53,03,54,04,55,0

Grain size (µm)

Welding speed (mm/s) Heat Input rate (J/mm)

Minimum point 183,26 µm (HI=85,16 J/mm ; V=3,1 mm/s)

(7)

Minimum grain size correspond to the value of 183.26µm. The optimum welding parameters for the 1.2 mm thick FSS are the following: welding current about 55A at the welding speed of 3.1 mm/s.

The presence of martensite is a cause of weakness because it reduces the ability of plastic deformation. It is good to subject the assemblies to thermal treatment (for FSS is at about 800 ° C) that transforms the martensite to alpha ferrite and carbides (Figure 5).

Fig.5: Fusion zone after heat treatment at 800 ° C.

The curves of the microhardness of the weld have the same shape (figure 6). After heat treatment of specimens, we see a decrease in hardness in the FZ and the HAZ and stabilize in the Base Metal(MB).

Fig.6: Hardness before and after weld heat treatment (load 300g).

CONCLUSION

The results obtained show that, the microstructural characteristics of the weld are influenced by the current and the welding speed. The grain size can be reduced by reducing heat input during the welding process. Energy input can be reduced by accelerating the thermal cycle (rise of welding speed) or by reducing the welding current.

Heat treatment reduces the presence of martensite and ameliorates the resistance of the assembly.

RÉFÉRENCES

[1] Monypenny JHG. Stainless iron and steel. 3rd Ed. London: Chapman and Hall Ltd.; 1954.

[2] Pickering FB. The Metallurgical evolution of stainless steels. Int Met Rev 1976; 1.

200µm

Grain boundary

20µm

0 50 100 150 200 250 300

5- 4,5- 4- 3,5- 3- 2,5- 2- 1,5- 1- 0,5- 0 0,5 1 1,5 2 2,5 3 3,5 4 4,5

Hardness (HV) Before treatement

After treatement

distance( mm) distance( mm) distance (mm)

(8)

le Soudage, le CND et l’Industrie des Matériaux et Alliages (IC-WNDT-MI’12) Oran du 26 au 28 Novembre 2012, http://www.csc.dz/ic-wndt-mi12/index.php 73 [3] Hedge JC. Arc Welding Chromium Steel and Iron. Metal Progress 1935; 27 (4):33-38.

[4] Amuda MOH, Mridha S. Grain refinement in ferritic stainless steel welds: the journey so far.Adv Mater Res 2010; 83:1165-1172.

[5] M.O.H. Amuda , S. Mridha “Comparative evaluation of grain reﬁnement in AISI 430 FSS welds by elemental metal powder addition and cryogenic cooling” Materials and Design (2012), 35 609–618.

[6] Reddy GM, Mohandas T. Welding aspects of ferritic stainless steels. Indian Welding Journal 1974; 27 (2): 7.

[7] Brando WS. Avoiding Problems when welding AISI 430 Ferritic Stainless Steel. Weld Int 1992; 6:

713.

[8] G. Mallaiah, A. Kumar, P. Ravinder Reddy, G. Madhusudhan Reddy « Influence of Grain Refining Elements on Mechanical Properties of AISI 430 Ferritic Stainless Steel Weldments -Taguchi Approach » Materials and Design (2011), 10.1016

[9] Mohandas T, Reddy GM, Naveed MD. A comparative evaluation of gas tungsten and shielded metal arc welds of a ferritic stainless steel. J Mater Process Technol 1999; 94:133-140

[10] Easterling, K.E., 1993. Introduction to The Physical Metallurgy of Welding, second ed. Butterworth- Heinemann, Oxford.

[11] Easterling K. Introduction to the physical metallurgy of welding. 2nd ed. Oxford: Butterworth Heinemann; 1992. p. 18.