How Does Rapid Cooling Affect the Structure of Laser Welds in Stainless Steel?

A previous article in this newsletter entitled “Laser Welding Austenitic (300 Series) Stainless Steel” introduced the effects of composition and microstructure on the tendency for laser welds in austenitic stainless steels to crack.

The article also noted:

“Cooling rates associated with laser welding, which are generally high with respect to other welding processes, also influence the structure of the weld metal and its corresponding tendency to crack.”

It is generally recognized that the heating and cooling rates associated with laser welding are greater than those with arc welding processes upon which much of the welding information in handbooks and on the internet is based.

The higher heating and cooling rates arise from the characteristics of the laser beam as well as from the application. Laser energy is highly concentrated and welding speeds are relatively fast. Laser welds generally involve a relatively small volume of material affected by the laser process.

As the importance of laser welding in manufacturing continues to grow, so does the knowledge of the process and of properties of laser welds. This article highlights the fact that guidelines for laser welding may differ from those for arc-welding.

Microstructure and cracking

Arc welds containing 3-8% delta ferrite in an austenite matrix are expected to be crack-free whereas a purely austenitic structure is prone to cracking. The same rules for microstructure and cracking tendency are expected for laser welding. In other words, a laser weld that solidifies as purely austenitic is likely to include cracks whereas a weld with 3-8% ferrite content is expected to be crack-free.

Differences in the cooling conditions between arc welds and laser welds result in differences in the compositions (Cr/Ni Equivalent) that lead to the desired amount of delta ferrite.

The room temperature microstructure changes significantly with cooling rate during solidification of the weld. Studies have shown that an alloy that will solidify with an optimal amount of delta ferrite during arc-welding, will solidify as primary austenite (crack prone) when laser welded because of the higher cooling rates.

The effect of cooling rate is illustrated in the chart below.

Cr equivalent (ferrite forming tendency) = Cr + 1.37Mo + 1.8Si + 2Nb + 3Ti
Ni equivalent (austenite forming tendency) = Ni + 0.31Mn + 22C + 14.2N + Cu
Diagram showing the effects of composition and cooling rate on cracking of austenitic (300 series) stainless steels. Source: Lippold, J.C., “Solidification Behavior and Cracking Susceptibility of Pulsed-Laser Welds in Austenitic Stainless Steels”, Welding Research Supplement, June 1994, pp. 129-s through 139-s.

The curve on the left shows the original Suutula diagram denoting the effect of composition on cracking for arc welding. Compositions that yield Cr/Ni equivalent ratios to the left of this line, are likely to crack. However, those to the right of the line, or with a Cr/Ni equivalent greater than 1.48, do not crack.

For laser welding, this curve is shifted to the right (to Cr/Ni equivalents greater than 1.67) where the weld solidifies as a mix of the delta ferrite and austenite that promotes a crack-free structure.

How is this information used?

The Prima Power Laserdyne Application Engineer uses this information as a guide to selecting materials to be laser welded and, in some cases, to selecting filler material that will achieve a crack-free composition of the weld metal. This analysis is relevant to both similar and dissimilar metal combinations.

Example: Determining the Cr/Ni Equivalent for a Common 300 Series Stainless Steel

An internet search shows the following composition for 304L stainless steel, one of the most widely used of the austenitic stainless steels:

Weight %
Carbon 0.03 max.
Manganese 2.00 max.
Phosphorus 0.045 max.
Sulfur 0.030 max.
Silicon 0.75 max.
Chromium 18.0-20.0
Nickel 8.0-10.5
Nitrogen 0.10 max.
Iron Balance

If we use the maximum values for elements for which a maximum is indicated and the mean value for those for which a range is indicated, the Cr and Ni equivalents can be calculated as follows.

Cr equivalent = Cr + 1.37Mo + 1.8Si + 2Nb + 3Ti
= 19.0 + 1.37*0 + 1.8*0.75 + 3*0 = 20.35
Ni equivalent = Ni + 0.31Mn + 22C + 14.2N + Cu
= 9.2 + 0.31*2.00 + 22*0.03 + 14.2*0.1 + 0 = 11.9

Cr/Ni equivalent ratio = 20.35/11.9 = 1.71

The value of S + P + B = 0.075

When plotted on the graph above, we see that this composition is in the ‘No cracking’ zone.