Welding of Ultra-Fine-Grained Steel and its Hardenability Characteristics
ABSTRACT
The increase of resistivity of high strength steel has posed challenges to welding process in many welding and fabrication applications. The use of ultra-fine-grained (UFG) steel eliminates or reduces the need to increase the micro-resistance value while preserving or even increasing the strength properties, which has been a difficult task to achieve. UFG steel is a high-strength, low-alloy steel with a grain size of less than 1 to 2 microns. Compared with traditional steels, the strength of UFG steel increases significantly with a considerable decrement of its ductility. This article discuss the welding process for UFG steel and its hardenability characteristics in an effort to show its potential use as a miniature fabrication material in some industry.
Keywords: Ultra-fine-grained steel, Special welding method, Hardenability Characteristics
INTRODUCTION
High strength steel has been widely used in the fields of shipbuilding, heavy trucks, civil engineering, and structural engineering due to its good strength-to-weight ratio and excellent corrosion resistance [1]. However, most high-strength steel with a yield strength greater than 655 MPa limits the application range in welding due to their associated difficulties to achieve a successful welding process, such as the significant increase in resistivity, resulting in low weldability [2].
The resistivity of traditional high-strength steel increases rapidly with the increase of strength, making welding process very challenging. This issue has been circumvented by introducing alloying elements, such as Ni, Cr and Nb, that increase the strength of traditional high-strength steel while significantly reducing their micro-resistance value. Such steel, although with good strength, often falls short in terms of deformation and ductility critical for structural reliability [3].
As an alternative way to improve the micro-resistance of high-strength steel and its associated welding process, UFG steel was introduced. It is characterized by a micro-structure composed of significantly smaller grain sizes (less than 1 to 2 microns) as compared to traditional steel. This ultra-fine-grained structure results in an increased volume fraction of second-phase particles, providing excellent strength and hardness properties [4]. UFG steel provides the possibility to reduce or even eliminate the need for the addition of alloying elements. Additionally, UFG steel is believed to have potential for use in miniature structures where traditional steel cannot be applied.
WELDING TECHNIQUES
The micro-structure of UFG steel is expected to significantly affect the welding techniques and result in larger deformations during welding due to the increased influence of grain boundaries, second-phase particles, and residual stresses as compared to traditional steel. This can be addressed by the adoption of special welding techniques or the use of pre-treatment prior to welding.
One of the most effective techniques to join UFG steel welds is High Frequency Electrical Resistance Welding (HFERW). HFERW leads to the formation of solid-state and welding residual stresses that provide an increase of the effective yield strength and a higher toughness and fatigue life [5]. Like HFERW, Laser Welding is another welding process that can be safely used for joining UFG steel. However, unlike HFERW, very deep welds and wide weld beads with fewer pores and cracks can be achieved with Laser Welding, possibly due to the lower heat input and more homogeneous heat affected zone [6]. In addition, for specific welding requirements of UFG steel, Special welding processes, like use of filler wire, such as Nickel-based filler, that can eliminate or reduce the need to increase the micro-resistance value while preserving or even increasing the strength properties can be used [7]. Preheating and post-weld heat treatment, such as tempering at 350-550°C and water or air cooling, or even combination of preheating and tempering, are expected to alleviate the welding residual stresses and to soften the weldment [8].
HARDENABILITY CHARACTERISTICS
The hardenability of steel is a property that describes the ability of steel to harden in depth under a given set of conditions. In UFG steel, hardenability is affected by the grain size. Due to the small grains of UFG steel, the ferrite grain boundaries occupy the majority of the volume, and the maximum hardness is limited by the carbides precipitation along the grain boundaries. A combination of controlled cooling and post-heating treatment can further improve the hardenability of UFG steel by enhancing the formation of fine carbides along the grain boundaries. Additionally, prior to the heat treatment, the UFG steel should be normalized or spheroidized at a temperature above the Ac1 and followed by homogenization to improve its hardenability.
CONCLUSION
This paper discusses the welding process and hardenability characteristics of UFG steel. It shows that UFG steel offers a potential new material for miniature structure fabrication. Special welding techniques and pre-post treatment are needed for successful welding process with UFG steel. The hardenability of UFG steel is affected by the grain size and can be improved with homogenization, normalization and post-heating treatment.
ACKNOWLEDGMENT
The authors thank the support from the National Natural Science Foundation of China and the Open Projects Program of the State Key Laboratory of Advanced Metals and Processing.
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