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

Influencing variables for the cold cracking behavior of steels 

  • 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 is considered by the carbon equivalent CET. It should be noted that the carbon equivalent of the base material is only used 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 taken into account.

The hydrogen in the weld metal and in the heat-affected zone mainly originates from hydrogen-containing components of the weld filler metals and welding consumables. In addition, hydrogen may enter the weld metal through moisture present on the workpieces, e.g. condensation water.

A very effective measure to avoid cold cracks is therefore preheating, which delays the cooling of the weld area during and after welding to give the hydrogen an opportunity for effusion [3]. Care should therefore be taken to ensure that the recommended minimum preheating temperature is maintained throughout the welding process. The temperature is normally measured on the workpiece surface facing the welder at a distance from the longitudinal edge of the weld groove of A = 4 * workpiece thickness (but not more than 50 mm) [4]. This must be used for workpiece thicknesses up to a weld seam thickness of 50 mm. If the thickness exceeds 50 mm, the required temperature shall be present at a minimum distance of 75 mm in the base metal in each direction for weld preparation, unless otherwise agreed. In addition, the heat input should be selected high enough, especially in the root layer, to avoid very small bead cross sections and extreme hardening. Especially with thick-walled workpieces, it is advisable to weld the joint in one heat. If interruptions are unavoidable, cooling should be delayed and preheating should be carried out again [5]. In multi-layer welding, the interpass temperature instead of the preheating temperature is becoming increasingly important. Preheating before welding the first bead can be dispensed with if the subsequent bead is welded in the heat of the first bead so that the interpass temperature does not drop below the preheat temperature required for cold crack resistant welding. Preheating shall not be performed if the maximum permissible cooling time t8/5 for the welded joint would be exceeded for a given energy per unit length and sheet thickness. This time must be observed in particular when welding thin plates and for position and back-up welding [6]. 

The content of diffusible hydrogen, converted to standard conditions of 0 °C and 1.013 bar, is specified in cm3/100 g of applied metal (HD) or in cm3/100 g of weld metal (HF). Detailed provisions on the determination of the diffusible hydrogen content in the weld metal can be found in DIN 8572, Parts 1 and 2 [7].

Characteristic hydrogen contents of the weld metal, determined in accordance with DIN 8572, are shown for some welding processes in the table below as examples [8]. Translated with www.DeepL.com/Translator (free version)

Characteristic hydrogen content and its evaluation (DIN 8572) 
Welding process Hydrogen content HD Evaluation
Arc welding with    

Stick electrode B 

 > 5 to <= 10 

<= 5

 low 

very low

Stick electrode R 

 approx. 25 high
Stick electrode C  approx. 40 low very high
GMAW <= 5 very low
SAW > 5 to <= 10 

<= 5

 low 

very low

The residual stresses of a welded construction depend on the material, the welding conditions and the structural design. The risk of cracks occurring in the welded joint as a result of internal stresses is particularly high when the weld cross-section is only partially filled. The residual stress level can be favourably influenced by selecting a weld metal which is not too strong and by the shape of the seam and the welding sequence. 

The heat input during welding can be regarded as another main influence on the properties of the welds. It influences the temperature-time cycle that takes place during welding. If necessary, the heat input value Q can be calculated as follows [9].  

Q = k (U * I) / v [kJ/mm] 

Unless otherwise specified, the thermal efficiency of welding processes (k) shall be based on the following table.

Thermal efficiency of welding processes
Process Faktor k
SAW with wire electrode 1,0
MMAW 0,8
MIG-welding 0,8
MAG-welding 0,8
Metal arc welding with cored wire electrode 0,8
Metal active gas welding with cored wire electrode 0,8
Metal inert gas welding with cored wire electrode 0,8
Metal active gas welding with metal filled wire electrode 0,8
Metal inert gas welding with metal filled wire electrode 0,8
TIG-welding 0,6
Plasma welding 0,6

Altogether the preheating causes a changed heat transport by heat conduction and leads thus

  • to prolong the cooling time and thus to a modified microstructure and to more favorable diffusion and effusion conditions for the hydrogen, and 
  • to change the residual stress state in the welded joint or in the component. 

The relationship between the various influencing variables and the minimum preheating temperature is described in the adjacent formula [10]. The carbon equivalent CET in %, the plate thickness d in mm, the hydrogen content HD of the weld metal in cm3/100 g and the heat input in kJ/mm shall be specified. It should be noted that if the carbon equivalents of base material and pure weld metal differ, the higher value determined must be taken into account.

Equation

T0 = 700 CET + 160 tanh (d/35) + 62 HD0,35 + (53 CET - 32) Q - 330

The above formula applies to steels with yield strengths up to 1000 N/mm2, with a carbon equivalent CET of 0.2 to 0.5%, a plate thickness d of 10 to 90 mm, a hydrogen content of HD 1 to HD 10 and a heat input Q of 0.5 to 4.0 Kj/mm. Further boundary conditions are [2], [3]: 

The interlayer temperature does not fall below the minimum preheating temperature and does not exceed 300 °C. 

  • Single layer fillet welds, tacking and root beads have a minimum length of 50 mm. If the plate thickness exceeds 25 mm, tack and root beads are welded in two layers using a less strong weld metal.
  • In the case of filling layer welds, this also applies to multi-layer fillet welds, no intermediate cooling takes place as long as the weld thickness is less than one third of the plate thickness. Otherwise, a low hydrogen annealing shall be performed. 
  • The welding sequence shall be selected such that excessive plastic deformation of the only partially filled weld is avoided. 

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 Schmelzschweißen, 4. Ausgabe, April 1993, Verlag Stahleisen, Düsseldorf 
[4] DIN EN ISO 13916:
Anleitung zur Messung der Vorwärm-, Zwischenlagen- und Haltetemperatur
Deutsches Institut für Normung, November 1996 
[5] Stahl-Eisen-Werkstoffblatt 063:
Empfehlungen für das Umformen und Schweißen von Stahlrohren für den Bau von Fernleitungen, 1. Ausgabe, April 1987, Verlag Stahleisen, Düsseldorf 
[6] Merkblatt DVS 0918:
Unterpulverschweißen von Feinkornbaustählen, Okt. 1988, DVS-Verlag GmbH, Düsseldorf 
[7] DIN 8572, Teil 1 + 2:
Bestimmung des diffusiblen Wasserstoffs im Schweißgut, Teil 1: Lichtbogenhandschweißen, Teil 2: Unterpulverschweißen Deutsches Institut für Normung, März 1981 
[8] Beiblatt zum Stahl-Eisen-Werkstoffblatt 063:
Empfehlungen für das Umformen und Schweißen von Stahlrohren für den Bau von Fernleitungen; Kaltrißsicherheit, 1. Ausgabe, April 1987, Verlag Stahleisen, Düsseldorf 
[9] DIN EN 1011-1: Empfehlung zum Schweißen metallischer Werkstoffe, Teil 1: Allgemeine Anleitungen für Lichtbogenschweißen, Apr. 1198, Beuth Verlag GmbH, Berlin 
[10] Uwer, D. und Höhne, H.:
Ermittlung angemessener Mindestvorwärmtemperaturen für das kaltrißsichere Schweißen von Stählen. Schweißen und Schneiden 43 (1991), Heft 5, S. 282 - 286