ANALYSIS OF CRACKS RESULTING FROMTHERMITE WELDING OF CATHODIC PROTECTION

Marjan Suban, Simon Božiè, Andrej Zajec, Robert Cvelbar, Borut BundaraInstitute of metal constructions, Ljubljana, Slovenia

ConclusionsIn the case of thermite welding of cathodic protection some problems connected to presence of liquid-solid metal contact (copper-steel) were detected. In welding area larger crystal grains, martensite microstructure and consequently, the microcracks appear, which are filled with liquid copper. To reduce this unfavourable microstructure, it is necessary to reduce the rate of cooling, which can be achieved by preheating the steel. By preheating to temperature at least 40 °C, reduction of martensite microstructure can be achieved, hardness at weld edge is lowered and initiation of microcracks is suppressed. This preheating also reduce moisture, which may appear on the surface of pipe and causes some other welding defects.In conclusion, some calculations were made for static strength showed that only minor reduction of strength can be expected. In the case of dynamic load, the crack propagation of filled crack is slower that in the case of empty crack. However, filled crack can also lead to material collapse.

Introduction

Thermite welding of cathodic protection

ResultsMacroscopic examinations

Microscopic examinations

Various steel pipes that are exposed to corrosion, are protected with the cathodic protection where thermite welding of a copper conductor on a steel pipe is used. During the welding process and due to the nature of it, steel in the solid state comes in a contact with liquid copper. Contact of steel with liquid metal in some cases cause phenomenon known as the liquid-metal embrittlement or LME. Phenomenon was previously studied in cases such as soldering, but for thermite welding no records were found in accessible literature. The purpose of this investigation was to draw attention to some irregularities and consequences arising from it, which in this type of welding can occur.

Thermite welding, also known as exothermic welding, of copper to steel is a welding process for joining these two materials, that employs superheated copper to permanently join the welding parts. The process takes advantage of an exothermic reaction of a copper thermite composition to heat and melt the copper. Thermite welding process itself is shown in Fig. 1. This type of welding is especially useful for joining dissimilar metals e.g. Cu and steel for creating electric joints, like in our case for cathodic protection.

Figure 1: Steps of thermite welding procedure

The welding setup shown in Fig. 1 was used for the laboratory tests. Copper conductor NYY 1x16 mm was welded on 24’’, SCH 40 steel pipe made of ASTM A106 Gr. B (P255G1TH). Cadweld exothermic system was used for welding. Tests were performed with new and multiple-times used moulds. Preheating temperature of steel pipe was set to:

o o o oroom temperature (15 C), 40 C, 60 C and 80 C.

In the macroscopic investigations the following characteristics of welded joints were observed: shape of the weld, porosity of the weld (Fig. 2a), lack of joint with steel base (Fig. 2b) and size of the heat affected zone (HAZ) in the base metal (Fig. 2c).

a) b) c)

Figure 2: Some examples of defects revealed by macroscopic examination

The copper and steel formed intermediate layer, which contains the penetrated copper. The thickness of this layer is from 10 to 20 mm.

Figure 3: Measurements of thickness of Cu-Fe fusion zone (intermediate layer)

With the presence of martensite microstructure in HAZ just below the fusion zone cracking occurs. This phenomenon (LME) occurs due to contact of liquid metal (Cu) with a solid metal (steel). Fracture is facilitated by adsorption-induced weakening of interatomic bonds at crack tips, with transport of copper to crack tips occurring rapidly (crack growth rate is up to 100 mm/s) by capillary flow. Diffusion of atoms along grain boundaries ahead of cracks is not involved, although this can occur in some circumstances.

Figure 4: Microcracks in steel filled with copper

Micro-hardness measurements just below the weld edge showed that the values in the martensite can be as high as 367 HV (Fig. 5 top). Such result was obtained in the case of over-used welding mould and at a temperature of steel pipe approximately 15 °C. Fig. 5 shows that by using of new mould and at minimum preheating, reduction in hardness just below the weld edge can be achieved. Reduced local hardness is result of absence of hard martensite microstructures. Also microcracking cannot be found in these cases.

Figure 5: Micro-hardness results in weld cross-section

Calculation of reduction of static strength of steel pipe due to the presence of filled crack can be done using equations derived by Panasyuk et al., where comparison of strength of material with empty crack and material with filled crack is made. If crack is filled with material of almost equal strength, that just a little reduction of static strength can be expected. In this research work crack propagation due to fatigue was not investigated, but some other research articles can be found in literature. Microcracks in steel, which are filled with other metal (Cu or Ni)lead to retardation of crackpropagation, as shownon Fig. 6.

Figure 6: Crack length growth versus cycle for cracks filled with Ni (EN5) and Cu (EC3)

Measurements of micro-hardness

Reduction of material strenght due to microcracks