Question

Choose the correct statement(s) in the context of fusion welding of austenitic stainless steel containing about 0.06 wt.% carbon.

• A. Corrosion resistance of heat affected zone is poorer than base material.
• B. Corrosion resistance of heat affected zone is superior than fusion zone.
• C. Corrosion resistance of heat affected zone is same as fusion zone.
  (D) Corrosion resistance is same for fusion zone, heat affected zone, and base material.
• D. Corrosion resistance is same for fusion zone, heat affected zone, and base material.

Hint:

For standard austenitic stainless steel welding: - Remember the acronym {HAZ} = {H}as {A} {Z}one of poor corrosion resistance. - This is due to sensitization (chromium carbide precipitation) in the temperature range \(\sim\)450-850°C. - To avoid this, use low-carbon "L" grades (e.g., 304L) or stabilized grades (e.g., 321, 347).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The question addresses a critical issue in the welding of austenitic stainless steels, specifically the phenomenon of "weld decay" or sensitization. This issue is directly related to the carbon content and the thermal cycle experienced by the material during welding.
Step 2: Detailed Analysis:
- The Material: Austenitic stainless steels (like the 304 grade) derive their excellent corrosion resistance from a passive layer of chromium oxide on the surface. This requires at least 12 wt.% chromium to be dissolved in the austenite matrix. The steel in question has 0.06 wt.% carbon, which is a standard, non-low-carbon grade.
- The Welding Process: Fusion welding involves melting the base material. The region adjacent to the molten weld pool, which does not melt but is heated to high temperatures, is called the Heat Affected Zone (HAZ).
- The Phenomenon (Sensitization): When austenitic stainless steels with this level of carbon are heated into a specific temperature range (approximately 450-850°C), chromium atoms diffuse to the grain boundaries and react with carbon to form chromium carbides (Cr\(_{23}\)C\(_{6}\)). This process happens in the HAZ during the cooling part of the weld thermal cycle.
- The Consequence: The formation of these carbides depletes the chromium from the regions immediately adjacent to the grain boundaries. If the chromium content in these depleted zones falls below the critical 12 wt.%, the material loses its ability to form a stable passive film in those areas. This makes the grain boundary regions highly susceptible to intergranular corrosion. This phenomenon is called sensitization.
- Comparing the Zones:
- Base Material: Properly solution-annealed, the carbon is dissolved, and chromium is uniformly distributed. It has excellent corrosion resistance.
- Heat Affected Zone (HAZ): It experiences the sensitization temperature range, leading to chromium carbide precipitation and chromium depletion at grain boundaries. Its corrosion resistance is significantly degraded.
- Fusion Zone (Weld Metal): This zone melts and re-solidifies rapidly. This rapid cooling often suppresses the formation of chromium carbides, or if they form, they are finely dispersed within the grains rather than at the boundaries. Furthermore, filler metals used for welding stainless steel often have very low carbon content or are stabilized with elements like Nb or Ti to prevent sensitization. Therefore, the fusion zone typically has good corrosion resistance, often comparable to or better than the sensitized HAZ.
Step 3: Evaluating the Options:
- (A) Corrosion resistance of heat affected zone is poorer than base material. Correct. The HAZ becomes sensitized and susceptible to intergranular corrosion, while the base material is not.
- (B) Corrosion resistance of heat affected zone is superior than fusion zone. Incorrect. The sensitized HAZ has poor corrosion resistance compared to the typically non-sensitized fusion zone.
- (C) Corrosion resistance of heat affected zone is same as fusion zone. Incorrect. They have different microstructures and corrosion properties.
- (D) Corrosion resistance is same for fusion zone, heat affected zone, and base material. Incorrect. The welding thermal cycle creates distinct microstructural zones with different corrosion properties.
Step 4: Why This is Correct:
The primary metallurgical issue with welding standard carbon austenitic stainless steels is sensitization of the HAZ, which specifically degrades its corrosion resistance compared to the unaffected base metal. Therefore, statement (A) is the only correct description of the situation.