Efficient Welding of stainless steel to High-Alloy and Low-Alloy carbon Steels: Tips and Techniques
Welded joints between unalloyed and low-alloy structural steels and austenitic chromium nickel steels are colloquially referred to as black-white joints. Black stands for structural steel (CS: carbon steel), white stands for stainless steel (SS: stainless steel). There are numerous factors to take into account in order to produce a reliable welded joint with the optimally selected filler metal.
A key challenge in these welds is the potential compromise to the Stainless Steel's corrosion resistance due to the presence of carbon on the Carbon Steel side. Additionally, significant disparities exist in the physical and mechanical properties of Stainless Steel and mild Carbon Steel, necessitating caution to avoid a martensitic weld metal that is prone to brittleness. Additionally, temperature and time-dependent structural changes can occur, such as sigma phase embrittlement.
On the contradict of welding un- and low alloyed steels here the filler metal is choosen by the highest alloyed base material
An important factor for getting the right weld metal is dilution. When we weld dillution occurs between the base material(s) and the filler metal. Dilution is expressed in percentage (%) and depends on welding process and the welding parameters.
or
| SAW - Strip | 15% -25% |
| ES - Strip | 5%- 15% |
| SAW- Wire | 40% - 50% |
| Electrode | 15% - 30% |
| Mig / MAG | 25% - 40% |
| Tig with filler metal | 20% - 40% |
| Tig no filler metal | 100% |
Typical B1 10% and B2 10% Filler metal F1 80%
Dilution rate depends of the welding process and to the welding parameters. Ideally, the dilution with the base material should be minimised, for example, by reducing the energy of the weld through optimised welding parameters. To achive that following must be kept in mind.
- Low amperage, voltage and high welding speed to achieve a low heat input
- Welding in stringer beads
- Use of small electrode diameters
- Controlled preheat- and interpass temperatures.
Please note that the arc should not be directed at the ferritic base material, but at the already melted weld metal
The selection of the right filler metal is depending on many things: type of material, operating temperature and the corrosion recistance. The following must be taken into account when welding Stainless Steel - Carbon Steel joints.
- Use welding process with low dilution and the right parameters
- With alloy type 18 8 Mn, there is no risk of hot cracking due to the increased Mn content
- No post-weld heat treatment and use up to a maximum of 300°C in operation (as the formation of a Cr carbide zone and a decarburized zone is to be expected at higher temperatures, which reduces the strength!)
- Pay particular attention to low dilution
- Also suitable for application temperatures above 300°C
- Also suitable for post heat treatment
- Buffering the joint flank of the low-alloy base material with Ni-based alloy
- Carrying out the necessary post-weld heat treatment, e.g. tempering or stress relief annealing
- Welding the joint between the Ni-base buffer layer and the high-alloy material with nickel-based filler metal
In the literature you will find some help for an initial classification for the selection of the welding filler metals, a classification according to Prof. Hermann Thier is very suitable here.
| gr. | Characteristics of the exposure |
Problems | Filler metal Alloy type Material number ASME |
CEWELD product GMAW / Tig / Electrode |
| 1 | T < 300°C Mechanical stress No heat treatment |
Martensite formation Hardening cracks Hot cracks Toughness |
18 8 Mn / 1.4370 / 307 23 12 / 1.4332 / 309 23 12 Mo / 1.4459/ 309LMo 29 9 / 1.4337 / 312 |
CEWELD 307Si / 307 Si Tig / 4370 Ti CEWELD 309LSi / 309LSi Tig / 4332 Ti CEWELD 309LMo / 309LMo Tig / 4829 MoTi CEWELD 312 / 312 Tig / CroNi 29/9 S |
| 2 | T < 300°C Mechanical stress No heat treatment + Corrosion stress Possibly with heat treatment |
23 12 L Cap layer according to |
CEWELD 309LSi / 309LSi Tig / 4332 Ti CEWELD 309LMo / 309LMo Tig / 4829 MoTi |
|
| 3 | T > 300°C + Mechanical stress No heat treatment + Corrosion stress Possibly with heat treatment Or Alternating temperature stress |
Carbon diffusion Yield strength Heat resistance Stresses due to different thermal expansion coefficients |
Ni –Alloy e.g. NiCr20 Nb 2.4648 / 2.4806 / 2.4831 |
CEWELD NiCro 600 / NiCro 600 Tig / E NiCro 600 CEWELD NiCro 625 / NiCro 625 Tig / E NiCro 625 |
The Schaeffler diagram is a useful tool for describing the forming microstructure. As a general rule, the weld metal composition should be moved to less vulnerable areas. It is advisable to avoid the area with martensite content, as this is where embrittlement phenomena occur that can lead to cracking. The choice of filler metal is also restricted by the fact that the resulting weld metal should not be located in the austenite area, as there is a risk of hot cracking during solidification in the case of purely austenitic phase formation. If the resulting weld metal alloy is too far to the right in the Schaeffler diagram, sigma phase formation will occur, particularly at elevated temperatures during subsequent use. This is present in a very brittle form and also leads to a weld seam hazard. A relatively small area remains in the centre of the diagram.
When working with the Schaeffler diagram, its validity limits for the alloy proportions must be observed.
These are C < 0,2 % S i < 1 % Mn < 4,0 % Mo < 3 % Nb < 1,0 %
for example
| Nr | Materials used | W.Nr | C | Si | Mn | Cr | Ni | Mo | CrA | NiA |
| 1 | P310GH ( Ferrit ) | 1.0482 | 0,20 | 0.50 | 1,20 | - | - | - | 0,75 | 6,6 |
| 2 | X15CrNiSi25-21 ( Austenit) | 1.4841 | 0,15 | 2,0 | 1,60 | 25 | 21 | - | 28 | 26,3 |
| 3 | 23 12 L / 309L Filler metal | 1.4332 | 0,02 | 0,8 | 0,8 | 24 | 13 | - | 25,2 | 14,0 |
The points resulting from the nickel and chromium equivalents for base materials 1 and 2 are entered in the Schaeffler diagram (see figure) and connected with each other.Assuming that both base materials are melted in equal proportions, the centre of the straight line corresponds to the microstructure of the mixed base material (point A).
From the position of this point, it can be deduced that TIG welding would not be suitable without filler material. In this case, the point would correspond to the weld metal point and lie in the hot cracking danger zone.
The microstructure point 3 of the filler metal is also drawn in the diagram and connected to point A of the filler metal. The length of the straight line is now assumed to be 100% and the proportion of the filler metal for the welding process used (rod electrode approx. 20%) is subtracted from the filler metal side (point 3). This results in point B for the mixed structure = mixed weld metal (red).
For the above example, the application temperature of the component must also be taken into account, which must not exceed 300 °C. Nickel-based filler metals are preferable for higher operating temperatures. However, the position of the filler metal outside the Schaeffler diagram does not permit any calculation.
The combination of quenched and tempered steel and austenitic requires the use of nickel-based welding consumables due to the limited weldability of the quenched and tempered steels and the post-weld heat treatment that is generally required.In such cases, it is recommended that 3-layer cladding (buffering) of the weld flanks of the quenched and tempered steel and subsequent annealing is carried out.Exceptions to this are possible if no heat treatment is carried out. In such cases, genuine overalloyed welding consumables can be used, provided that an operating temperature of max. 400 °C is not exceeded.
The filler metal should be selected taking into account the chemical composition of the higher alloyed material partner. The Schaeffler diagram can be a valuable guide to minimise the risk of cracks.
This combination can also be checked using the Schaeffler diagram. The filler metal depends on the operating conditions. Austenitic filler metals must therefore be specified taking into account their tendency to embrittlement. In certain cases, the use of nickel-based filler metals may be necessary.
Weld filler metals should be used that correspond to the heat-resistant material partner in terms of alloy. The Schaeffler diagram is also helpful here.
Unfortunately, the Schaeffler diagram is not or only partially applicable here. There is a risk of embrittlement during annealing or at the usual operating temperatures. A nickel-based filler metal such as CEWELD E NiCro 600 or CEWELD NiCro 600 would be recommended here.
