Crack parameters When the carbon equivalent is equal to […]
When the carbon equivalent is equal to or less than 0.17 wt-%, or high-strength steel is used, the Ito & Bessyo parameter crack detection (Pcm) can be used. This method can accurately predict when to preheat, when to force preheat, and to what temperature to preheat. Specifically
If Pcm ≤ 0.15%, preheating can be selected arbitrarily
If 0.15%<Pcm< 0.26–0.28%, preheat to 200°–400°F (100°–200°C).
If Pcm> 0.26–0.28%, preheat to 400°–700°F (200°–350°C).
Spark inspection has been used for decades and is a method of estimating the amount of carbon in carbon steel. The higher the carbon content, the better the spark and the more preheating is required. Although this method is not very precise, it is simple. The relative height of the preheating temperature can be determined.
Rule of thumb
Another less precise but very effective way to select the preheat temperature is to calculate the preheat temperature increase by 100°F (50°C) for every 10 points of carbon content (0.10 wt-%). For example, if the carbon content is 0.25 wt-%, the preheating temperature is 250°F (125°C) or at least 250°F (125°C). If there are coatings or other components near the weld, the preheating temperature determined according to the original production specification is not appropriate. However, if the welding heat input is near the maximum range allowed by the standard process, then the heat transferred to the welding component may be equalized by the welding heat input, causing the affected metal to be heated to or exceeding the minimum preheating requirement, so , It can be preheated more loosely by external methods.
It should be noted that the range and imprecise conversion (°F to°C) is used here. It is intentional. Preheating is not a very precise science. In many cases, it is normal to continue to increase the preheating temperature until the problem is solved (such as cracks disappear). On the contrary, in some special occasions, even using a preheating temperature lower than the recommended value or the temperature required by the specification can achieve the purpose.