The Inter pass temperature during fillet welding shall not be permitted to drop below the ? A) minimum preheat temperature (B) maximum preheat temperature (C) all of the above (D) 300°C @all

• Preheat temperature is the temperature of the base metal immediately prior to welding. It's applied to slow the cooling rate and reduce the risk of hydrogen-induced cracking.
• Interpass temperature is the temperature of the weld area between the deposition of successive weld passes.

For effective control of cooling rates and prevention of defects, the temperature of the weld area should not be allowed to drop below the established minimum preheat temperature during subsequent passes. If it drops below the minimum preheat, the benefits of preheating are lost, and the risk of cracking increases. There is also often a maximum interpass temperature to prevent issues like excessive grain growth or distortion.

• A) minimum preheat temperature: This is consistent with standard welding codes and practices. The interpass temperature must be maintained at or above the minimum preheat temperature.
• B) maximum preheat temperature: The interpass temperature should generally not exceed the maximum preheat or interpass temperature, but it is certainly permitted to be below the maximum, as long as it stays above the minimum. The question asks what it "shall not be permitted to drop below."
• C) all of the above: Incorrect, as option B is not the correct lower limit.
• D) 300°C: This is a specific temperature that might be a maximum interpass temperature for certain materials, but it is not a universal minimum for all fillet welding and doesn't directly relate to the preheat concept in the same way as option A.

The correct answer is A.
$\boxed{\text{A}}$