One of the biggest problems in the welding of high-strength fine-grained steels is cold cracking. In general, the cold cracking tendency of micro-alloyed fine-grained steels is low [1]. However, if higher carbon contents are present, hydrogen-favored cold cracks can occur in the heat-affected zone. Since other alloying elements besides carbon also favour cold cracking, carbon equivalents are often used to estimate the crack sensitivity. There are numerous formulas for describing the carbon equivalent, in which the individual alloying elements are weighted differently.
The carbon equivalent can thus be generally understood as a measure of a material's tendency to cold cracking depending on its chemical composition. It also serves as a basis for calculating the minimum preheating temperature Tp and the cooling time t8/5, which are necessary to exclude cold cracking after cooling of the weld seam. 

The cold cracking behaviour of welded joints depends mainly on the influencing factors given in the table on the right [2].

  • Chemical composition 
  • Workpiece thickness in the seam area 
  • Hydrogen content of the weld metal 
  • Heat input during welding 
  • residual stress level of the construction 
  • Preheat temperature/interlayer temperature 

The influence of the chemical composition on the cold cracking behaviour of steels can be described sufficiently precisely by the carbon equivalent CET. 
This results in limit values up to the thickness of which steel sheets with the corresponding chemical composition can be welded without preheating, if normal welding conditions are applied and a favourable residual stress condition is present [3].

Carbon equivalent CET [%] max. plate thickness without preheating [mm]
0,18 60
0,22 50
0,26 40
0,31 30
0,34 20
0,38 12
0,40 8

It should be noted that the permissible plate thickness is only determined by the carbon equivalent of the base material 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 increased by a safety margin of 0.03 % shall be used to determine the permissible plate thickness.

CET

The carbon equivalent CET was formulated by Uwer und Höhne in 1991 [4] and is currently the most comprehensive carbon equivalent for the prevention of cold cracking. The range of validity refers to the permissible ranges of the individual alloying elements given in brackets on the input page [5].

Equation: 

CET = C + (Mn + Mo)/10 + (Cr + Cu)/20 + Ni/40 

CE

The carbon equivalent CE goes back to a publication of the International Institute of Welding (IIW) more than 20 years ago [6]. It is based primarily on hardness measurements and was derived under the assumption that alloying elements that contribute to hardening promote cold cracking to the same extent. Since the carbon equivalent CE, compared to newer carbon equivalents, strongly underestimates the effect of carbon, it is less suitable for the treatment of cold cracking problems than newer models [4]. It is not suitable especially in the area of short cooling times.

Equation: 

CE = C + Mn/6 + (Cr + Mo + V)/5 + (Cu + Ni)/15 

PCM

The carbon equivalent PCM is based on Japanese results from Ito and Bessyo in 1969 [7]. It can be used for short cooling times and root welding [8].

Equation: 

PCM = C + Si/30 + (Mn + Cu + Cr)/20 + Mo/15 + Ni/60 + V/10 + 5*B 

CEM

The carbon equivalent CEM can only be used 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) [8].

Equation: 

CEM = C + Si/25 + (Mn + Cu)/20 + (Cr + V)/10 + Mo/15 + Ni/40 

CEN

The carbon equivalent CEN developed in Japan is a purely mathematical combination of the carbon equivalents CE and PCM [9]. However, it is no better suited to describe the cold cracking behavior than the underlying carbon equivalents CE and PCM.

Equation:

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)  

Literature

[1] Dilthey, U.:
Schweißtechnische Fertigungsverfahren Band 2, Verhalten der Werkstoffe beim Schweißen, 2. Auflage, 1995, VDI Verlag, Düsseldorf 
[2] Uwer, D. und Wegmann, H.:
Anwendung des Kohlenstoffäquivalents CET zur Berechnung von Mindestvorwärmtemperaturen für das kaltrißsichere Schweißen von Baustählen, DVS-Jahrbuch Schweißtechnik 96, Deutscher Verband für Schweißtechnik, S. 46 - 55 
[3] Stahl-Eisen-Werkstoffblatt 088:
Schweißgeeignete Feinkornbaustähle, Richtlinien für die Verarbeitung, besonders für das Schweißen, 4. Ausgabe, April 1993, Verlag Stahleisen, Düsseldorf 
[4] Uwer, D. und Höhne, H.:
Charakterisierung des Kaltrißverhaltens von Stählen beim Schweißen. Schweißen und Schneiden 43 (1991), Heft 4, S. 195 - 199 
[5] Uwer, D. und Höhne, H.:
Ermittlung angemessener Mindestvorwärmtemperaturen für das kaltrißsichere Schweißen von Stählen. Schweißen und Schneiden 43 (1991), Heft 5, S. 282 - 286 
[6] Technical Report 1967, IIW Doc. IX-535-67 
[7] Ito, Y. und Bessyo, K.: 
Weldability Formula of High Steels, Related to Heat-Affected Zone Cracking, Sumintomo Search, 1 (1969), H. 5, p. 59 -70 
[8] Düren, C.: 
Konzepte zur Bewertung des Kaltrißverhaltens von Stählen - Beispiele im Bereich der Großrohrstähle, 3R international, 28. Jahrgang, Juli 1989, Heft 6, S. 385 - 391 
[9] Yurioka, N. et. al.: 
Study on Carbon Equivalents to Asses Cold Cracking Tendency and Hardness in Steel Welding, Australian Weld. Res. Ass. Melboure 19. - 20.03.81, Paper 10, p. 1 - 18