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Summer 2019 Contents: Issue No. 43
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Solidification Cracks in Welds
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Cracks are the most detrimental of all defects that can occur in a weld. One of the most common forms of cracks are those that form during solidification as the molten metals cools down to room temperature. While the parent metal is at least macroscopically homogenous (there may be fine distribution of secondary particles) and made so by prior processing, the weld fusion zone is not. As the weld cools down from molten metal, solidification produces segregation based on solubilities and melting points of different phases in the weld. Combination of segregation, low melting phases, and distortion from cooling stresses can result in solidification cracks.
It is quite surprising how even extremely small amount of impurity elements can cause phase segregation and cracking. A good example to get an understanding of this issue is the Iron (Fe)-Sulfur (S) system. Iron is the primary constituent in steels and sulfur occurs mostly as an impurity element controlled at below 0.03 wt. %. However, in machinable grade steels, sulfur content can be as high of 0.30 wt.%; ten times nominal levels. Even at nominal 0.03 wt. %, amount of sulfur is more than can be dissolved in solid phases such as austenite and ferrite.
Let us take a hypothetical situation where a Fe-S alloy with 0.03% S was welded with a fusion process (arc/laser/ebeam). At the end of the weld, the weld fusion zone would start to solidify from the outer boundaries through which heat is conducted away by the parent metal. Solid grains in the parent metal right next to the molten metal would start growing by adding atoms on their surface; a weld does not need special nucleation sites as would be the case in a casting. The solid metal grains growing inwards (see Figure 1), initially in a columnar fashion, can only take in a very limited amount of sulfur in solution; any un-dissolvable sulfur would get pushed out to the fast-growing grain boundaries and toward the liquid metal in the center of the weld. As more and more sulfur gets pushed out, the percentage of sulfur in the remaining liquid keeps growing to a point where a thin liquid mixture of iron and iron sulfide (Fe-FeS) remains. Such a eutectic mixture has a melting point of about a 1000°C, much lower than that of iron at 1500°C (See Figure 2), or melting point of FeS at 1200°C.
During solidification and cooling, there will also be distortions and flexing of the fast-growing grains due to thermal contractions (based on Coefficient of Thermal Expansion). Combination of segregation and cooling effects can lead to gaps between grains which, if not filled by the eutectic liquid, will appear as cracks. Solidification cracks typically show up as throat cracks along the centerline, longitudinal cracks along the welding seam, crater cracks on the last spot of a seam weld, and as separation/crack/pores along the weld interface in resistance welds. Such cracks can also form deep inside the weld and may not be obvious on the surface. See Figure 3 for examples.
Fortunately for us, the steel industry figured out early on that adding Manganese (Mn) to the Fe-S mix can result in Mn preferentially reacting with S to for Manganese Sulfide which has a high melting point, and importantly keeps S from forming the low melting Fe-FeS eutectic. Practically all steels have a minimum of 0.3-0.5% Mn, about 10 times the amount of S, explicitly to reduce/prevent cracking during metal working (hot-shortness) and during welding (solidification cracking). Of course, these problems will not exist if steels were free of S, but doing is not practical at any reasonable cost.
Solidification cracks can occur in many other systems, not just steels with sulfur. They can occur in Nickel (Ni) – Phosphorous (P) systems, and are commonly encountered in welding parts with electroless Ni plating which can have up to 15% P. Copper alloys in the bronze family are also susceptible to solidification cracking and so are Aluminum alloys such as 6061.
Listed below are some of the options to eliminate solidification cracking based on specific weld chemistry, geometry, and process:
- Welds in 316 can have cracks when cooled rapidly as in laser welding due to presence of dominant austenite phase which has almost no solubility for sulfur; alternative is to swap one of the components to 304 which has a small fraction of ferrite which can absorb some amount of sulfur.
- Slowing down the cooling rate as with TIG seam welding instead of laser pulsed welding can also avoid cracking in 316.
- Avoid using machinable steels which have up to 0.3 wt. % sulfur. Using corresponding non-machinable grades are more expensive for machining but much better for welding as sulfur is limited to 0.03%.
- For resistance welds, the contaminant phases will be flushed towards the weld centerline and appear as a crack along the weld interface. Option is to apply a forging force at the end of the weld cycle to close the cracks.
- If the alloy cannot be changed, the best option is to produce convex weld profile in arc/laser/ebeam welds. Such a convex profile can be created by using features in the part geometry or by adding filler. A convex bead will produce compressive stresses on cooling which will help close any cracks that may have formed.
- In aluminum welds, where such solidification cracks cannot be easily avoided, the option is to flood the weld with low melting eutectic fluid that will fill all the cracks that form during cooling. An example would be to use 4043 filler to weld a 6061 component. If filler application is not convenient, one of the parts to be welded can be made of 4043 alloy.
Figure 1. Schematic weld section showing internal cracks and surface cracks formed during solidification.
Figure 2. Iron (Fe) – Sulfur (S) phase diagram. Fe-FeS eutectic has a lower melting point of 1000°C; lower that both Fe and FeS.
Figure 3. Weld sections showing from left to right: a throat crack, a longitudinal crack on surface of a seam weld, and separation inside a resistance weld nugget.
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