Why carbon content is important?
The most widely used metal material in the industry is carbon steel and its alloys. Carbon steel is compounded based on carbon content and alloying elements. carbon content is one of the elements with the most influence on the nature or characteristics of a metal. The more carbon content, the metal becomes stronger and harder. however, hard material has brittle characteristics or is less resilient and increases the likelihood of cracking in the weld joint area.
Explanations of the carbon equivalent
One of the biggest problems in the welding technology of high-strength fine-grained steels is cold cracking. In general, the cold cracking tendency of micro-alloyed fine-grained steels is low. However, if higher carbon contents are present, there may be hydrogen-favored cold cracking in the heat affected zone. Since other alloying elements promote cold cracking in addition to carbon, carbon equivalents are often used to estimate crack sensitivity. There are numerous formulas for describing the carbon equivalent, in which the individual alloying elements are weighted differently.
The carbon equivalent may thus generally be understood as a measure of the inclination of a cold cracking material depending on its chemical composition. It also serves as a basis for the calculation of the minimum preheating temperature Tp and the cooling time t8/5, which are necessary in order to exclude a cold cracking after cooling the weld can.
Factors influencing the cold cracking behavior of steels:
- Chemical composition
- Workpiece thickness in the seam area
- Hydrogen content of the weld metal
- Heat input during welding
- The residual stress level of the construction
- Preheating / interpass temperature
The influence of the chemical composition on the cold cracking behavior of steels can be described sufficiently accurately by the carbon equivalent CET. This results in limit values up to the thickness of which steel sheets with appropriate chemical composition can be welded without preheating if standard welding conditions are used and a favorable residual stress state is present.
It should be noted that the permissible sheet thickness depends on the carbon equivalent of the base material only if the carbon equivalent of the weld metal is at least 0.03% lower than that of the base material. Otherwise, the carbon equivalent of the weld metal, which has been increased by a safety margin of 0.03%, will be used to determine the permissible sheet thickness.
The carbon equivalent CET was formulated in 1991 by Uwer and Höhne and currently represents the most comprehensive carbon equivalent for preventing cold cracks. The scope of validity refers to the permissible spans of the individual alloying elements given in brackets on the input side.
CET = C + (Mn + Mo) / 10 + (Cr + Cu) / 20 + Ni / 40
The carbon equivalent CE is based on a publication published more than 20 years ago by the International Institute of Welding (IIW). It is primarily based on hardness measurements and has been derived on the assumption that alloying elements that contribute to hardening promote aliasing to the same extent. Since the carbon equivalent CE greatly underestimates the effect of carbon compared to newer carbon equivalents, it is less suitable for the treatment of cold cracking problems than newer models. It is not suitable in particular in the area of short cooling times.
CE = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15
The carbon equivalent PCM is based on Japanese results of Ito and Bessyo in 1969. It can be used for short cooling times and root welds.
PCM = C + Si / 30 + (Mn + Cu + Cr) / 20 + Mo / 15 + Ni / 60 + V / 10 + 5 * B
The carbon equivalent CEM is only under the very limited conditions of the short cooling time range (2 to 6 s) and the narrow validity range of the chemical composition (C: 0.02-0.22, Si: 0.00-0.50, Mn: 0) , 40 – 2.10, Cu: 0.00 – 0.60, Cr: 0.00 – 0.50, Ni: 0.00 – 3.50, Mo: 0.00 – 0.50, V: 0 , 00 -0.10) usable.
CEM = C + Si / 25 + (Mn + Cu) / 20 + (Cr + V) / 10 + Mo / 15 + Ni / 40
The carbon equivalent CEN developed in Japan represents a purely mathematical combination of the carbon equivalents CE and PCM. However, to describe the cold cracking behavior, it is no better suited than the underlying carbon equivalents CE and PCM.
CEN = C + (0.75 + 0.25 * tanh (20 * (C-0.12))) * (Si / 24 + Mn / 6 + Cu / 15 + Ni / 20 + (Cr + Mo + V + Nb) / 5 + 5 * B)
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