Hydrogen, carbon equivalent and preheating — why steel cracks
Cold cracks, the CE formula and what the electrode says
Introduction
Cold cracks are cracks that can appear hours after welding, when the weld has cooled down. Three things work together: how hardenable the steel is, how much hydrogen there is in the weld metal, and how fast the weld cools. How fast it cools depends on the material thickness and on how much heat you put in.
The blacksmith training regulations have the learning objective: «The apprentice knows how to calculate carbon equivalents and their significance».
The carbon equivalent CE
The carbon equivalent combines the steel's content of carbon and alloying elements into one number. The higher the number, the more hardenable the steel, and the greater the risk of cold cracks. The most widely used formula is the IIW one:
- 01The formula applies to steels with a CE between 0.30 % and 0.70 %
- 02If only C and Mn are on the certificate, add 0.03 for the other elements
- 03If you join two different steels, use the higher CE
- 04The formula may not suit steels containing boron
Worked example
Example with our own figures: C 0.18 · Mn 1.40 · Cr 0.05 · Mo 0.02 · V 0.01 · Ni 0.05 · Cu 0.05. CE = 0.18 + 0.233 + 0.016 + 0.007 = 0.44.
The manufacturers state the CE for their steel. SSAB states for S355J2+N a maximum of 0.45 and typically 0.42 (6-60 mm). Ovako states a maximum of 0.47 for S355J2. The figures differ because they come from different manufacturers — always check the certificate for your particular steel.
Hydrogen
Hydrogen in the weld metal is measured in ml per 100 g of weld metal. ISO/TR 17844 divides it into five hydrogen classes:
| Hydrogen (ml per 100 g weld metal) | Class |
|---|---|
| More than 15 | A |
| More than 10 and at most 15 | B |
| More than 5 and at most 10 | C |
| More than 3 and at most 5 | D |
| At most 3 | E |
Rutile and cellulosic electrodes belong to class A. Basic electrodes can be in classes B to D, depending on the manufacturer's classification. Solid wire for MIG/MAG and TIG is normally regarded as class D.
What does it say on the electrode?
ESAB's basic electrode OK 48.00 is classified as «E 42 4 B 42 H5» according to ISO 2560. This is how you read parts of the designation:
- 0142: the weld metal has a yield strength of at least 420 MPa, a tensile strength of 500-640 MPa and an elongation of at least 20 %
- 024: the impact toughness is at least 47 J at −40 °C
- 03B: basic covering
- 04H5: the hydrogen class. The data sheet states less than 4.0 ml of hydrogen per 100 g of weld metal
The second «42» says something about recovery, type of current and positions. We do not explain it here — see the electrode's data sheet.
Heat input
Heat input is how much energy the arc puts into each millimetre of weld. It is measured in kJ/mm. High heat input gives slow cooling: less risk of hard zones, but coarse grains and more distortion. Low heat input gives fast cooling and a greater risk of cracks. The formula and the efficiency factors are in EN 1011-1. Use the calculator, and stick to the heat input your WPS specifies.
Preheating
Preheating makes the weld cool more slowly. Whether you need to preheat, and by how much, is stated in the WPS. It is not a free choice on the shop floor.