Do you know, during welding process, base metal and filler metal treated like "heat treatment" process in phase diagram (e.g. austenite, martensite, etc) , What's impact this behavior on wedling? how this process happen? Can it cause defect? how to avoid it? please read furthermore in below article.
Due to the fact that during welding the base metal is always heated to above Ac1 or Ac3 respectively in specific areas of the heat-affected zone, there is always a danger with hardenable steels of hardness increase and as a result crack formation. The tendency towards hardness increase with non-alloy and alloy steels depends in particular on the carbon content but also on the content of other alloys. During welding the speed of cooling from the austenitic range may be so great that it corresponds approximately to hardening in water.
The cooling speed becomes greater
◆ the less heat is inputted during welding,
◆ the thicker the material is,
◆ the colder the material is.
If the critical cooling speed is reached, one must reckon with the formation of hardened micro structures , e.g. martensite. The level of the hardness values is largely determined by the carbon content. The hardness increases linear to the rise in carbon content up to approximately 0.45 % C to a value of around 650 HV. The impact energy in the hardened steel lies above 78 Joule up to 0.12 % and above this drops off sharply. Above 0.2 % C it lies below 32 Joule. From this one can see that the value of 0.2 % C approximately represents the limit up to which steels can be welded without preheating and without the need for special precautions.
If the filler and cover passes are then welded over this root pass, the zones lying below them are
normalised or tempered and the hardness peaks next to the root weld are reduced. However, if
cracks have already occurred beforehand in the transition zone due to postweld hardness increase, then they also remain after applying the cover passes to the weld and may possible lead to fracture of the welded component.
In zones hardened in this way high stresses are set up due to the effect of welding shrinkage because the material is prevented from reducing them by means of plastic deformation. Over and above this a multi-axis stress condition is set up in this area particularly in the case of thick crosssections which is promoted still further due to martensite formation taking place with increase of volume. Cracks occur in the transition area if the stresses reach the cohesion strength.
Hydrogen may also be significantly involved in the occurrence of these underbead cracks. If possible a hardness value of 350 HV should not be exceeded in order to prevent these underbead cracks with some degree of certainty. Accurate knowledge of the hardening processes in the heat-affected zone of the steel is especially important to prevent underbead cracks and for the safety of a welded structure for the reasons mentioned above. It also appears to be very important prior to welding to be able to predict a possible hardness increase for a specific steel with a known chemical composition.
Due to the fact that during welding the base metal is always heated to above Ac1 or Ac3 respectively in specific areas of the heat-affected zone, there is always a danger with hardenable steels of hardness increase and as a result crack formation. The tendency towards hardness increase with non-alloy and alloy steels depends in particular on the carbon content but also on the content of other alloys. During welding the speed of cooling from the austenitic range may be so great that it corresponds approximately to hardening in water.
The cooling speed becomes greater
◆ the less heat is inputted during welding,
◆ the thicker the material is,
◆ the colder the material is.
If the critical cooling speed is reached, one must reckon with the formation of hardened micro structures , e.g. martensite. The level of the hardness values is largely determined by the carbon content. The hardness increases linear to the rise in carbon content up to approximately 0.45 % C to a value of around 650 HV. The impact energy in the hardened steel lies above 78 Joule up to 0.12 % and above this drops off sharply. Above 0.2 % C it lies below 32 Joule. From this one can see that the value of 0.2 % C approximately represents the limit up to which steels can be welded without preheating and without the need for special precautions.
If the filler and cover passes are then welded over this root pass, the zones lying below them are
normalised or tempered and the hardness peaks next to the root weld are reduced. However, if
cracks have already occurred beforehand in the transition zone due to postweld hardness increase, then they also remain after applying the cover passes to the weld and may possible lead to fracture of the welded component.
In zones hardened in this way high stresses are set up due to the effect of welding shrinkage because the material is prevented from reducing them by means of plastic deformation. Over and above this a multi-axis stress condition is set up in this area particularly in the case of thick crosssections which is promoted still further due to martensite formation taking place with increase of volume. Cracks occur in the transition area if the stresses reach the cohesion strength.
Hydrogen may also be significantly involved in the occurrence of these underbead cracks. If possible a hardness value of 350 HV should not be exceeded in order to prevent these underbead cracks with some degree of certainty. Accurate knowledge of the hardening processes in the heat-affected zone of the steel is especially important to prevent underbead cracks and for the safety of a welded structure for the reasons mentioned above. It also appears to be very important prior to welding to be able to predict a possible hardness increase for a specific steel with a known chemical composition.
No comments:
Post a Comment