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cooling rate
- Thread starter aybee
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The prevention of cracking during cooling of carbon steel welds is a rather complex problem. Several important variables are carbon equivalent of the metals you are joining, preheat temperature, welding process, interpass temperature and weld filler metal chemical composition (including dissolved hydrogen).
Cracking can occur in the hardened heat affected zone of carbon steel base metals that exhibit a carbon equivalent above 0.4 with no preheat. The application of preheat slows the rate of cooling to avoid the formation of martensite (which is a harmful microstructure because it can become brittle with increasing carbon content in steels) in the base metal HAZ's.
The first approach to solving your problem is to use the aforementioned CCT program to project a theoretical maximum cooling rate that would avoid the formation of martensite in the heat affected zone of the base metals. You would need to project a cooling rate that would intersect the nose of the transformation curves to the right in the CCT diagram.
A theoretical cooling rate can be calculated to factor the effect of preheat versus no preheat using Rosenthal's Equation for 2-dimensional heat flow for bead on plate. I had recently purchased one of the best reference books on welding - "Welding Metallurgy" by Sindo Kou that provides extensive information on this topic.
The heat flow equation that you would use is as follows;
delta (T/t) in deg/sec = -2 *pi*k*V(T - To)^2/Q
k = work piece thermal conductivity
V = travel speed
T = Arbitrary temperature like 500 deg C or higher as a reference point
To = initial temperature of the work piece
Q = heat transferred from heat source to work piece (need to know the arc efficiency)
As you can see, when you apply preheat and increase the value of To, the delta (T/t) decreases in value, and you can project this cooling rate on a CCT diagram to avoid the formation of martensite.
Cracking can occur in the hardened heat affected zone of carbon steel base metals that exhibit a carbon equivalent above 0.4 with no preheat. The application of preheat slows the rate of cooling to avoid the formation of martensite (which is a harmful microstructure because it can become brittle with increasing carbon content in steels) in the base metal HAZ's.
The first approach to solving your problem is to use the aforementioned CCT program to project a theoretical maximum cooling rate that would avoid the formation of martensite in the heat affected zone of the base metals. You would need to project a cooling rate that would intersect the nose of the transformation curves to the right in the CCT diagram.
A theoretical cooling rate can be calculated to factor the effect of preheat versus no preheat using Rosenthal's Equation for 2-dimensional heat flow for bead on plate. I had recently purchased one of the best reference books on welding - "Welding Metallurgy" by Sindo Kou that provides extensive information on this topic.
The heat flow equation that you would use is as follows;
delta (T/t) in deg/sec = -2 *pi*k*V(T - To)^2/Q
k = work piece thermal conductivity
V = travel speed
T = Arbitrary temperature like 500 deg C or higher as a reference point
To = initial temperature of the work piece
Q = heat transferred from heat source to work piece (need to know the arc efficiency)
As you can see, when you apply preheat and increase the value of To, the delta (T/t) decreases in value, and you can project this cooling rate on a CCT diagram to avoid the formation of martensite.
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