Silicon (Si)
Silicon binds oxygen and prevents porosity in the weld pool, increases strength and scale resistance, but too high a content leads to embrittlement and hot cracking.
Physical-Chemical Mechanism of Action
Silicon reacts with oxygen in the molten weld pool and forms silicate. This prevents the formation of pores because the oxygen is no longer available for reaction with the metal. Silicon also improves strength and scaling resistance through a better crystal structure.
Metallurgical Relevance & Welding Risks
A silicon content that is too high (above approx. 0.40 % in combination with carbon) leads to weld metal embrittlement and increases susceptibility to hot cracking. This can result in a fine, glass-hard structure in the matrix that can easily crack under mechanical stress.
Shop Practice & Action Guidelines
Make sure the silicon content in the filler materials is kept within spec. If it's too high, let the weld cool more slowly (e.g., t8/5 > 25 s) to get a less glass-hard structure. However, avoid keeping the silicon too low, because that can lead to porosity. A silicon content between 0.30 % and 0.40 % is recommended for most steel grades.