The hardness issue of pipes
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For 20# steel pipes, hardness testing is required according to the design specifications after heat treatment. After testing, it was found that the hardness of the base material was around 160 HB, while that of the heat-treated area was around 120 HB. The requirements specified in SH3501-2011 state that the hardness of carbon steel after heat treatment should not exceed 220 HB. So the question arises: is it still necessary to test the hardness of 20# steel? Whether to fight or not meets the design requirements – isn’t it just a waste of money? Which sea friend can share some experience?
Testing the hardness after heat treatment is meaningful for determining the degree of annealing. It is not necessary to test the hardness in the heat-affected zone, as it contains both coarse-grained and fine-grained areas, making it impossible to determine the hardness accurately.
Then there must be a problem with the standards I’ve decided to follow, or I haven’t yet understood the true intent behind those standards. Based on experience, the hardness of a weld decreases after heat treatment (tempering); since the hardness of the base material itself already meets the specifications, it is pointless to measure its hardness again after heat treatment. In other words, for 20# steel, it is not necessary to conduct a hardness test after heat treatment.
20 steel is generally subjected to stress-relief annealing treatment (with the base material thickness usually being 30 mm or more), as there is a certain relationship between strength and hardness. The purpose of annealing heat treatment is to eliminate stress and restore mechanical properties, with hardness testing used to evaluate the effectiveness of the heat treatment.
It still needs to be done... You can think of it as a follow-up check···
Those in the design field prioritize rigor and safety above all else.
For 20# steel, other standards specify that the thickness should be greater than 19. . . .
Well, for some low-temperature steels, such as those of type A333-6, it’s not possible to achieve hardness in the heat-affected zone either
The purpose of annealing heat treatment is to eliminate stress and restore mechanical properties, with hardness testing used to evaluate the effectiveness of the heat treatment. =========================== It does not eliminate stress, but rather eliminates the peak stress generated by the welding heat cycle
It depends on the specific requirements; it may be necessary to measure the hardness or not. For example, in our company, we don’t measure the hardness of 20# steel