Often, the base metal is preheated before welding. What are the specific reasons for this?
Let's start with the conclusion: There are two main reasons:
The first is to reduce residual stress in the weld; the second is to reduce the formation of hardened structures in the weld, thereby reducing the occurrence of cold cracking during welding.
How can we understand this conclusion?
Let me give you an example: Before exercising, we usually warm up, mainly to stretch our muscles and joints, so that we are less likely to suffer muscle strains during exercise.
In fact, our steel also needs some warm-up exercises before welding. Preheating is a good warm-up exercise.
Steel is usually subjected to relevant heat treatment before leaving the factory. However, even after heat treatment, there will still be residual stress in the steel. Therefore, preheating the steel before welding can minimize this residual stress.
This residual stress is called constraint stress in engineering, which is essentially the same thing.
For welding cold cracking, there are three main factors: constraint stress, hardened structure, and the content of diffusible hydrogen.
Constraint stress is one of the influencing factors.
The second aspect, as we just discussed, is to reduce the formation of hardened structures.
Martensite is a typical hardened structure; it is a supersaturated solid solution of C in α-Fe. Martensite is also divided into lath martensite and acicular martensite.
Generally, lath martensite has relatively better properties, while acicular martensite has poorer properties. But I won't go into detail here; we can discuss the topic of martensite later if there's an opportunity.
However, for welding, we should try to avoid the formation of martensite in the weld.
You can see the image above. This image is a typical isothermal transformation curve diagram for eutectoid steel.
In reality, the CCT curve for welding is much more complex. For ease of understanding, we will temporarily use this diagram to explain how martensite is formed.
In the diagram above, the five lines from V1 to V5 represent different cooling rates. V1 has the smallest slope, so the cooling rate is the slowest. V5 has the fastest cooling rate. In the lower part of this diagram, there are two curves, Ms and Mf. Ms represents the temperature curve at which martensitic transformation begins, generally around 230 degrees Celsius. Mf represents the temperature curve at which the martensitic transformation ends.
The probability of martensite formation varies with different cooling rates. V5 represents the fastest cooling rate, and it is the most likely to produce martensite.
Therefore, preheating the steel before welding increases the welding temperature. This prolongs the cooling time of the welded joint, thus reducing the cooling rate and consequently decreasing the probability of martensite formation.
This is one of the main reasons why preheating is necessary before welding.
So, is a higher preheating temperature always better? Of course not!
If the preheating temperature is too high, the cooling time of the weld will be extended. Especially in the temperature range of 800 to 500 degrees Celsius, if the cooling time is too long, grain growth will be more pronounced.
The strength and hardness of the weld will decrease significantly. Therefore, the preheating temperature should not be too high.