International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –

6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 36-43, © IAEME

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METALLURGICAL BEHAVIOUR OF AISI 304 STEEL BUTT WELDS

UNDER SEGREGATION

Rati Saluja1, K.M. Moeed

2

1*

Department of Mechanical Engineering, Goel Institute of Technology & Management, Lucknow,

UP, India 2Department of Mechanical Engineering, Integral University, Lucknow, UP, India

ABSTRACT

Correlation is to integrate depictions of fundamental microscopic phenomena of

solidification, cooling, grain growth, segregation, metallurgical developments and its significance on

mechanical properties for AISI 304 steel. When formulating the susceptibility of to cracking,

segregation is not generally accountable, despite being a significant factor. Segregation is originated

by incomplete mixing in the weld pool which promotes the dendrite envelope motion governed by

the growth of dendrite tips. The growth model for dendrite tips can be obtained by solute transport

near the tip progressed by tip stability. Due to high segregations of chromium and molybdenum,

secondary ferrite is known to be susceptible to the formation of intermetallic phases such as sulfur or

carbon. But segregation of nickel, chromium and molybdenum can be lowered with the use of

nitrogen in the shielding gas. Competing factors add to the intricacy of the phenomenon, hence the

main objective of this scanty literature is concerned with metallurgical behavior of this class of

engineering materials under the influence of segregation.

Keywords: Austenitic, Dendrites, Ferrite, Segregation, Supercooling

1. INTRODUCTION

Selection of Type 304 steel as material of choice for a variety of engineering applications

such as structural steel parts cannot be traced only to its good corrosion resistant and formability

characteristics [1]. It is highly preferred due to its good welding characteristics, low cost, easy

availability in the market places this class of materials at the forefront of metallurgical technology

[2].

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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –

6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 36-43, © IAEME

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The AISI 304 steel has atleast of eight elements, and the use of binary and ternary phase

systems to value higher order systems, which can lead to significant error, while predicting

microstructures [3]. Furthermore, the σ phase usually precipitates in Grade 304 steels at 800℃ and

that it has a strong effect on the properties of stainless steels [4]. σ phase has significant influence on

material properties, it causes chromium impoverishment near the precipitates and reduces the

corrosion resistance [5]. It has also been demonstrated that the σ phase does not affect the

mechanical properties of stainless steels at 1000℃ because it dissolves in the matrix at this

temperature [6]. Grade 304 steel series of austenitic stainless steels usually solidify during welding

as a mixture of austenite and ferrite. The ferrite almost fully transforms to austenite on cooling, but

there could be retention of a few percent of δ ferrite in the weld metal which leads to cracking

processed by segregation [7]. This arises from larger solidification temperature range during primary

austenitic solidification. The inhomogeneity is affected also by considerably slower diffusion of

alloying elements in austenite than in ferrite [8].

Weld pool macro segregation occur by lack of weld pool mixing especially in welding of

dissimilar metals, or some special types of rapidly solidified power metallurgy alloys. It also occurs

during incomplete weld pool mixing in single pass welding (greater extent) and even in multipass

welding [9]. When comparing the segregation behaviour for different solidification modes it has

been demonstrated that microsegregation is stronger in welds having primary austenitic solidification

[10].

To this end, the fundamental study of the microstructural development of Grade 304 steel

presented here provides a baseline for comparison when the more complicated material behavior

under segregation. The work presented in this paper represents the first step in understanding the

complex solidification behavior of 304 steel, which are routinely used as parent metals for various

fabrications in metallurgical, mechanical, chemical, automobile and nuclear industries process. The

data gathered here will serve as the foundation for the continuing investigation of the intricate

material behavior under rapid solidification conditions that can potentially be induced during various

arc welding processes.

2. WELDING METALLURGY OF GRADE 304 STEEL WELDS

2.1 Solidification and cooling Under rapid solidification, a great number of welds having duplex microstructures are

replaced by single phase austenitic or single phase ferritic microstructures in arc welding. According

to Schaeffler diagram, in welds predicted to have less than 8% ferrite will transform to fully

austenitic with cooling rates of as high as 200000 K/s [10]. At high ferrite content approximately

more than 30% ferrite, arc welds predicted to have may become fully ferritic with these very high

cooling rates. Massive transformation to austenite in these welds may, however, abruptly decrease

the ferrite content [11,12].

The primary solidifying phase incorporates more solute than it does closer to equilibrium

conditions promotes which further restrain the formation of second phase ferrite in AF-mode and

second phase austenite in FA-mode [13]. Due to rapid solidification and retarded diffusion in FA-

welds solid state transformation from ferrite to austenite is repressed [10, 14]. The difference

between microstructure of fusion zone resulting from Type A, AF and F mode solidification can be

easily distinguished as shown in figure 1 [15].

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –

6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 36-43, © IAEME

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Fig. 1 Microstructure of fusion zone resulting from Type A, AF and F mode solidification [15]

The growth of primary austenite dendrites or cells enhances as solidification rate and dendrite

tip undercooling increases [16]. As an optical micrograph shows the change

in dendrite morphology from cellular to dendritic as the growth velocity increases toward the center

of spot weld towards top after the spot weld arc is extinguished.

Fig. 2 Micrograph of dendrite morphology from cellular to dendritic [14]

2.2. Microstructure Development and its effects on mechanical properties The welding metal contains a skeletal and lathy ferrite type dendrite structure as shown in

figure 3. It shows the morphology of the base material, which evidence the austenitic grain

microstructure of an AISI 304 stainless steel [17]. The lathy ferrite microstructure can emerge due to

greater ferrite contents and/or a characteristic cooling time after the welding procedure steel [18].

Fig. 3 Micrographs of the microstructures at the Fusion zone [19]

In AISI 304 steel, the ferrite-promoting element segregation specially chromium promotes

formation of ferrite stringers during solidification [19]. The final microstructure consists of γ

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –

6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 36-43, © IAEME

39

dendrites with a Molybednum concentration gradient and interdendritic α phase [20]. The presence

of Molybdenum rich liquid present near the termination of solidification wets the surface of the

solidified dendritic structure promotes formation of cracking [21]. As austenite is more susceptible to

this phenomenon than ferrite due to the lower solubility of tramp elements in austenite as in steel that

form low melting point solids [22]. However, this is not an option, as a fully austenitic structure must

be maintained to conserve the magnetic and corrosion resistance properties of Grade 304 steel.

The change in scale morphology can be recognized to an edge effect phenomenon occurring

during the cooling of the AISI 304 austenite stainless steel. Fig. 5 (a), (b) and (c) shows the stereo

micrograph of WM, HAZ and FZ which could be discriminated easily. In welding zone, a small

amount chromium carbide precipitation is formed. Whereas the microstructure analysis revealed that

the ferrite to austenite formation contribute in increasing the weldment strength, whereby at

weldment area ferrite form occurred due high temperature during welding. This makes the structure

strength at weldment area weaker compare to HAZ and base metal [23].

Fig. 4 Micrographs of the Microstructures of PM, HAZ and Weldment [24].

2.3. Segregation of Alloying Elements

Solute undercooling predicts that solute is rejected from the solidifying metal into the liquid

ahead of the solid liquid interface, resulting in a depression of the liquidus temperature of the liquid

ahead of the interface due to solute buildup [25]. Due to continuous reduction in temperature till the

termination of solidification, the dendrites that had initially formed continue to grow into the

increasingly solute enriched liquid results a composition gradient within the dendritic substructure of

the material, generally referred to as coring [12]. But Researchers suggests that segregation of nickel,

chromium and molybdenum can be reduced with the use of nitrogen in the shielding gas. The

segregation of nickel remains, reasonably weak [26].Development of dendrites in a nickel-based

superalloy single-crystal weld has been demonstrated in figure 5.

Fig. 5 Scanning-electron micrograph showing the development of dendrites in a nickel-based

superalloy single-crystal weld [14]

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –

6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 36-43, © IAEME

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In the case of primary austenitic solidification and the segregation ratio above one indicates

interdendritic segregation for all major alloying elements [27]. Correspondingly,

the concentration of chromium and preferably molybdenum, at the dendrite cores decreases than in

the bulk composition. This indicates that the segregation tendency of chromium and molybdenum

reduces and that of nickel increases with respect to primary austenitic solidification [20]. In primary

ferritic solidification the value of segregation ratio for all other elements except for nickel is lower

than in primary austenitic mode [8]. At the end of solidification the last melt droplets can once again

solidify as ferrite, known as secondary ferrite. Due to high segregations of chromium and

molybdenum, secondary ferrite is known to be susceptible to the formation of intermetallic phases

such as S or C [9].

Therefore, during cooling, the segregation profiles developed in austenitic solidification

are not homogenised to the same extent as in ferritic solidification [28].

Fig. 6 Regions of fine scale dendritic, facetted and eutectic structure between primary dendrites [29]

Macrosegregation arising from dendritic solidification is defined as compositional

inhomogeneity on a scale larger than that of the dendrite arm spacings [29]. With a planar

solidification front, this would, indeed, be a fair first-approximation, but less so with cellular growth

and downright misleading with dendritic growth. The more removed is the topography of the

solidification front from planar, the more the enriched liquid is held within that texture [30]. In so

doing, it has also essentially removed the solute field ahead of the dendrite tips under the standard

range of solidification conditions [31] and, hence, has removed the mechanism for 'normal'

segregation. It is well known that the inhomogeneity of AISI 304 steel welds is greatly influenced by

microsegregation as well as by the partitioning of alloying elements between ferrite and austenite

during solid state transformation of delta ferrite to austenite.

This has an effect on the final degree of inhomogeneity especially in primary ferritic

solidification [22]. The enrichment of the interdendritic liquid in the mushy zone by especially

silicon reduces liquid density for the majority of steels, which consequently reverses the predicted

circulation currents. But steel compositions with addition of molybdenum and tungsten, may result in

residual liquid of increasing density and the inverse channel geometry in channel segregation [32].

For example, comparison of core and interdendritic (ID) manganese contents is shown in table.

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –

6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 36-43, © IAEME

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TABLE 1 Comparison of core (D) and Interdendritic (ID) Manganese contents [33]

4. CONCLUSIONS

AISI 304 steel transforms as alpha ferrite at the end of the solidification, which leads to

segregation followed by cracking. Evidence is presented to suggest for a range 304 steel series,

solidification, cooling rates, and secondary dendrite arm spacings under the influence of segregation

reveal the following.

1. During the solidification both micro segregation and macro segregation occurs in AISI 304

steel weldments due to presence of low alloys.

2. Microsegregation appears to be associated with the cracking in fisheyes. A small gas pore,

probably hydrogen, was found at the fraction initiation sites of each fish eye in the weld.

3. Macro segregation is found due to presence of manganese excessively in the composition

which results due variations in welding parameters under rapid solidification.

4. The solidus temperature can be reduced significantly with independent increases in either the

secondary dendrite arm spacing or cooling rate.

5. The solute-element concentration, especially phosphorus and sulfur, has a significant effect

on the solidus temperature the LIT and ZDT, due to their enhanced segregation near the final

stage of solidification.

6. Due to presence of Molybdenum rich liquid present near the termination of solidification in

the final microstructure consists of γ dendrites with a Molybdenum concentration gradient

and interdendritic α phase which results cracking.

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