Role of Nb in the failure of dual-phase steel in heterogeneous welds

https://doi.org/10.1016/j.engfailanal.2020.104708Get rights and content

Highlights

  • Nb-based intermetallics lead to hot cracking in two-phase stainless steel.

  • Delta ferrite affects the Nb-based intermetallics in two-phase steel.

  • Carbon redistribution can suppress sensitivity to hot cracking.

Abstract

The use of Cr-Ni austenitic steel interlayers is an effective solution for heterogeneous welds, especially in combination with high-carbon steels. High contact-fatigue resistance and resistance to impact loads are required in the case of railway crossing applications. The failure of Nb-alloyed cast Cr-Ni austenitic steel used as an insert in the flash butt resistance weld of Hadfield steel with carbon steel is the subject of the presented analyses. The identification of the failure mechanism is based on light and scanning electron microscopy and energy dispersive X-ray analyses, including electron backscattered analyses. The initiation of hot-crack defects is detected in the heat affected zone (HAZ). In contrast to the usual conditions for the formation of hot cracks in the HAZ, cracks are found solely outside the superheating zone. The decisive influence of Nb-based intermetallics formed at the phase interface of δ-ferrite and the austenitic matrix is indicated. The eutectics formation between niobium carbonitrides and the austenitic matrix is assumed to be responsible for the hot-cracking susceptibility of austenitic steel. The particular combination of the temperature gradient together with the chemical gradient leading to a localization of the critical zone is documented. The paper describes the influence of carbon redistribution at the weld interface on the morphology of delta-ferrite in different layers of the heat-affected zone. Induced structural differences are associated with Nb-particle formation. Fractographic analyses show the formation of low-melting phases in the critical layer of the HAZ. The influence of secondary phases, especially Nb-eutectic, on the degradation of austenitic steel plasticity is documented.

Introduction

Austenitic steels have a wide range of applications as a construction material based on their corrosion resistance, high-temperature strength, and thermal fatigue. Superior plasticity and thus capacity for stress relaxation are the preferential parameters for weldability, especially in the case of chemical and structural heterogeneous joints [1], [2], [3], [4]. Steels based on the Cr-Ni alloying group present a common solution for welding unalloyed high carbon steels with austenitic steels.

Surface welding as part of the maintenance of the worn contact surfaces of rail profiles and the flash butt welding of casted crossing parts are typical cases of heterogeneous welds in railway transport and are characterized by high dynamic loading requirements [5]. Flash-butt welding of common pearlitic steels and railway crossing track parts is widely used for railway crossings. Standard rail steel grades are pearlitic steels between 0.5 and 0.7% C, while the widely used steel for the most dynamic loaded switch parts is Hadfield austenitic steel. The mentioned material combination produces controversial requirements for the thermal welding conditions. While the limiting process of high-carbon steel welding is martensitic transformation due to the critical cooling rate in the heat affected zone (HAZ), and the common solution is preheating or increased thermal input, austenitic steels are sensitive to overheating and require limitation of the thermal input. The limiting process is carbon redistribution and embrittlement due to carbide formation along the grain boundaries. The established solution is butt resistance welding using an insert made of low carbon austenitic steel [6], [7], today in particular a Cr-Ni steel stabilised with niobium and/or titanium. The flash butt welding procedure includes two stages: (1) welding of the preheated carbon steel rail to the insert, followed by cutting off the insert to a length of about 20 mm; (2) welding of the high manganese Hadfield steel to the insert in a second welding operation.

The stable superior dynamic resistance of the entire welding joint, particularly dynamic fracture resilience together with rolling contact resistance, are important in this application. Microstructural weld variations that are responsible for mechanical property degradation are commonly caused by carbide precipitation and intermetallic phases formation [8], [9], [10]. The intermetallic phases most frequently encountered in austenitic stainless steels are sigma (σ) phase, chi (χ) phase and Laves phase [11], [12], [13]. Fully austenitic stainless steels are free of δ-ferrite and intermetallic phases such as sigma (σ) and chi (χ) phases. Fully austenitic stainless steels are free of δ-ferrite and intermetallic phases in their ideal form. In the case of the presence of δ-ferrite, as in the analysed type of insert stainless steel, the more complex phase transformation is very sensitive to the welding temperature gradient, and thus to the stress distribution and deformation after welding.

In this study, the subject of the performed analyses is the specific failure of the heterogeneous welds of the aforementioned type and applications in which Nb-alloyed steel is used for the insert part. Cracks occurred in all cast parts of the rail profile, i.e., without a significant effect of the specific width of the casting, except for the adjacent zone along the fusion line. The results of structural analyses and the circumstances of the formation of phases leading to the initiation of cracks are presented. The analyses are focused on local differences of Nb-phase formation, which are responsible for the failure of the welding joint even before the operational loading.

Another objective of the study was to identify the mechanisms that led to a defective fracture response, especially to the loss of the primary plasticity of the used stainless steel. The connection between the morphology of delta ferrite and the formation of intermetallic phases is studied. Chemical microanalyses combined with diffraction analyses are used to map the effect of the delta ferrite/austenitic matrix interface on the Nb phase distribution tendency. The effect of carbon redistribution at the weld interface was inspected. The cooling gradient difference in the damage zone compared to the intact zone is assessed with the support of temperature flow numerical simulation. Fractographic analyses of experimentally created fracture surfaces in various states of steel served to directly verify the influence of intermetallic phases on fracture behaviour.

Section snippets

Characterisation of failure

The heterogeneous welded Hadfield steel and carbon steel joint was made by flash-butt welding. The welding process was set to a pressure of 650 kN in combination with a current pulse up to 60A. Welding joint was created using an 25 mm thick insert of an Nb-alloyed austenitic cast steel. The final profile of the welded joint was machined according to the European Standard EN 13674–1 for rail profile UIC60. The chemical composition of the applied materials is given in Table 1. The damaged zone

Analytical methods employed

To identify the damage mechanism, a combined analysis comprising structural changes and phase composition detection together with associated fracture behaviour changes was performed.

The microstructure was analysed using optical microscopy (OM, using a NIKON SMZ 800, Neophot 32 and AnalySiS DOCU Olympus), scanning electron microscopy (SEM, using a TESCAN VEGA 5130SB) with energy dispersive X-ray spectroscopy (EDS, using a Bruker Quantanax 200). The combined EDS – EBSD (electron backscatter

Microstructure of the original material

The microstructure of the parent steel contains 15% delta-ferrite. Typical morphology of δ-ferrite islands is documented in Fig. 3a, b; continuous interconnected networks occurred only to a limited extent. Small carbide particle precipitation in the austenitic matrix follows the dendritic casting structure – Fig. 3c. Very fine intragranular precipitates were observed in the austenitic volume, except for the narrow zone along with the ferrite islands. The regions along the phase are clean,

Identification of the failure mechanism, discussion of the identified effects

The performed analyses together confirm the formation of so-called hot cracks as the initiation of a failure of the insert made of anti-corrosion Cr-Ni steel. The key moment is the formation of Nb-phases in response to local differences in carbon content and temperature gradient.

The presence of Nb-phases was identified as a crucial limiting point in heterogeneous weld failure, while the variability of their precipitation was driven by delta-ferrite morphology. The oblong precipitates formed a

Conclusion

It may be concluded that the decisive degradation effect leading to the failure of the heterogeneous welds was hot micro-crack formation. Some specific conditions of Cr-Ni steel degradation were observed in the analyzed case. The following causal link can be considered as a key failure process:

  • Formation of Nb-particles at the interface of δ-ferrite–austenite due to the lower solubility of Nb in austenite compared to delta-ferrite. The process is induced during the phase change of delta ferrite

Funding

This work was supported by the project No. SGS_2020_009 at University of Pardubice, Czech Republic.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References (26)

Cited by (0)

View full text