Welding Metallurgy of Stainless Steels

In order to understand the properties of iron based stainless steel weld metals and the associated phenomena that take place in the heat affected zone a knowledge of the constitution diagrams that relate to the alloying elements used in these alloys is required. Many welding phenomena can be explained and in many cases even predicted with the aid of constitution diagrams.

In steel, the cast structure of ingots is largely modified and positively influenced by hot forming and heat treatment operations, with processes of recrystallization, homogenization of segregations and the elimination of harmful precipitations through solution annealing playing a decisive role. In weld metals, such possibilities are rather limited, since welded components can only in rare instances be hot formed, solution annealed or quenched and tempered. In the untreated condition, weld metal normally emerges with a solidification structure and with previously executed passes of multilayer welds being exposed to a short reheating cycle in the course of depositing subsequent weld passes. But due to the steep temperature gradient, the high cooling rate and the short time span this reheating is in no way comparable to the controlled heat treatment processes of steels. Furthermore, only part of a previously laid pass is influenced in this way and the rest is maintained as a solidification structure.

After the solidification of stainless steel weld metal to primary crystals, mainly of a dendritic structure, during further cooling immediately recrystallization processes occur. New grain boundaries are formed which are no longer dendritic but are mainly globular or cellular in structure. Recrystallization is most pronounced in weld metals that do not undergo further transformation, i.e. weld metals which experience neither δ-γ nor γ-α transformation. These alloying ranges can be seen in Figs. 6 and 9 (pages 8, 10). They cover alloys with high chromium and low nickel contents which possess a high degree of delta ferrite. They are classified as ferritic or semi-ferritic stainless steel weld metals. The second range covers alloys with primary solidification to austenite which are classified as fully austenitic stainless steel weld metals.

As shown by the constitution diagrams which characterize metallurgical equilibrium, stainless steels possess at ambient temperature a very low solubility for carbon (see Chapter 1). Due to the high affinity of the major alloying constituent chromium towards carbon, there is a strong tendency towards carbide formation. The ranges of intermetallic phases, e.g. sigma phase, will often reach under equilibrium conditions far into the alloying ranges of stainless steels (see, for example, the iron-chromium system in Fig. 1 (page 2). Therefore we must expect the precipitation of these phases associated with the normal cooling rates experienced when working stainless steels, e.g. after rolling or forging. In most cases, however, they are undesirable because of their negative influence on both the corrosion resistance and the mechanical properties of the alloy.

Because of the great variety of possible types of cracks and causes for their formation, the problem of safeguarding against crack formation in welding operations is a highly complex one. A comprehensive survey of the cracking problems associated with the welding of different steel grades, is given by Baker [282] with special emphasis given to the different crack phenomena and the various causes of their formation. A survey of the different types of cracks found and a basic diagram of the temperature for possible crack formation in the welding of steel is also included in DIN 8524, Part 3. Because of their intricate appearance, hot cracks in austenitic welded joints are of particular importance and the literature available on this topic is quite substantial. Borland and Younger [283] compiled a survey of 162 publications up to the year ending 1959 regarding the hot cracking phenomena during the welding of austenitic chromium-nickel steels. In the documentation „Schweißtechnik, Bibliographie zum Thema Heißrisse beim Schweißen“ [284] which covers the years from 1968 to 1978, 81 out of a total of205 papers deal with austenitic materials. Another survey and a classification of hot cracks as they occur in the welding of various steels is supplied by Hemsworth, Boniszewski and Eaton [285]. A comprehensive survey on the present state of knowledge of heat affected zone (HAZ) cracking in thick sections of austenitic stainless steels is given with 91 references by Thomas Jr. [449].

Depending upon the alloying contents of chromium, nickel and carbon which are present in stainless steels, the structure after cooling to ambient temperature will consist of varying amounts of delta ferrite, austenite and martensite. Table 12 shows average values of the typical analysis of some weldable chromium steels, the carbon content of which has been set below 0.15% in order to improve the weldability. There are two main groups of steel, one with 12% and the other with 17% chromium, which differ mainly with regard to their corrosion resistance. With the addition of 1–2% molybdenum, the pitting resistance and retention of hardness can be improved. The corrosion behaviour of these grades, with special consideration towards welding is discussed by Bäumel [206, 207, 227] and by Honeycombe and Gooch [361].

According to Gysel, Gerber and Trautwein [369] the relatively poor weldability of chromium stainless steels, their cold cracking sensitivity and the often unsatisfactory mechanical properties obtained from welded joints led at the end of the fifties to the development of low carbon martensitic chromium-nickel steels. The basic idea behind this development was first of all the lowering of the carbon content in order to improve the toughness of the martensitic structure, to diminish the cold cracking sensitivity and to achieve a structure by adding 4–6% nickel which was as near as possible free from delta ferrite.

Common austenitic chromium-nickel stainless steels show only a limited resistance to stress corrosion cracking in chloride containing media when compared with ferritic chromium stainless steels with more than 18% chromium. However on the other hand ferritic chromium stainless steels—including the socalled “Super Ferritics”—are relatively difficult to weld (see chapter 6). During welding the predominantly ferritic structure shows a tendency towards coarse grain formation, particularly in the HAZ. With the carbon content of the common ferritic chromium steels in the range of 0.10% and above, an additional amount of brittle martensite is formed in the structure. Both phenomena cause a drop in toughness and an increased cold cracking sensitivity in the welded joint. The older types of austenitic-ferritic duplex steels containing 0.1–0.2% carbon were also unsatisfactory in terms of todays demands on the toughness and corrosion resistance of weldments, mainly because of their high susceptibility to intergranular corrosion (IC).

Stainless steels can generally be defined as steels with at least 11–12% chromium and carbon contents which are normally below 0.15%. With increasing demands on corrosion resistance, however, the chromium contents must be increased to above 16%. In ferritic stainless steels, the structure can be rather brittle which means poor weldability. Austenitic stainless steels show marked advantages here, particularly with regard to toughness properties and weldability. In stainless steels, the austenitic structure is generally produced by alloying with nickel. Nitrogen, a strong austenitizing element, has also gained increasing importance in recent years. One of the most important properties of austenitic stainless steels is their corrosion behaviour. The latter also determines their chemical composition. Beginning with the classical austenitic 18/8 chromium-nickel steel (AISI 302), Fig. 107 shows in a simplified but illustrative manner the effect that the basic elements and alloying additions have on the corrosion properties of the alloy, based on the representation of Sedriks [390]. For most types of corrosion an increase in the chromium content of the alloy above 18% will produce an improvement in corrosion resistance. A reduction in the carbon content to 0.030% max. or the stabilization of carbon by alloying with titanium or niobium will increase the alloys resistance against intergranular corrosion. Additions of molybdenum mainly improve the resistance to pitting and crevice corrosion.

When welding austenitic stainless steels the tendency towards greater distortion should be taken into consideration. Depending on the component design and dimensions, distortion can be counteracted and the need for straightening after welding reduced by following these guidelines:

Heat resisting steels are normally alloyed with chromium, silicon, aluminium and often also with nickel, with chromium being the most important element with regard to the scaling resistance. There exist ferritic, ferritic-austenitic und austenitic heat resisting steels. The latter contain higher amounts of nickel, normally in the range of approx. 4–35%. A comprehensive survey of the effect of alloying elements and scaling behaviour in different corrosive media is given by Rapatz [8], Houdremont [9], Fontana and Green [233] and Sedrics [390]. Table 24 shows some examples of the chemical composition requirements and scaling temperature limits of heat resisting steels. Compared to the corrosion resisting steels (see Tables 4, 12, 14, 16, 18, 20 and 22) not only are the silicon and aluminium contents increased but also the carbon content is often increased. If the ferritizing effect of 1% silicon and 1% aluminium is considered, these alloys will sometimes show high delta ferrite contents which in turn will lead to a strong tendency towards coarse grain formation at elevated operating temperatures. The ferritic heat resisting steels No. 1–3 thus show a strong tendency towards embrittlement which often appears only after cooling to ambient temperature. The presence of austenite counteracts coarse grain formation and such steels show less embrittlement than the ferritic grades No. 1–3. During operation, the fully austenitic heat resisting grades Nos. 5–9 also become coarse grained, however, they retain their toughness when cooled to ambient temperature because of their austenitic structure.

When welding stainless steel components, particularly in plant engineering applications, it may be necessary to join unalloyed or low alloyed ferritic steels to austenitic chromium-nickel-(molybdenum) stainless steels. Such dissimilar joints are also called “black-and-white joints”. With regard to corrosion resistance, there are normally no particular requirements demanded from this type of joint, because the low alloyed steel is normally not corrosion resistant. Because of the different structure between the types of steels to be joined, there may be certain difficulties experienced during welding. Low alloyed steels, for example, normally require controlled preheat and interpass temperatures during welding, slow cooling after welding and often a final tempering or stress relieving heat treatment. Austenitic steels on the other hand should be welded “cold”, the welds should cool rapidly and post weld heat treatments should be avoided wherever possible.