Microstructure and Mechanical Properties of Microalloyed Steels Containing Molybdenum

Jaqueline Polezi Mazini,Pedro Gabriel Bonella De Oliveira,Breno Mendes Rabelo Ávila,André Itman Filho,Rosana Vilarim Da Silva

doi:10.1590/1980-5373-mr-2021-0608

Jaqueline Polezi Mazini, Pedro Gabriel Bonella De Oliveira + Show 3 more

Open Access

https://doi.org/10.1590/1980-5373-mr-2021-0608

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Abstract

Microalloyed steels have a maximum of 2% in alloying elements and are used in the automotive and petrochemical industries. The microstructure and mechanical properties of these steels depend on chemical composition, processing and addition of molybdenum, niobium, titanium and vanadium. To evaluate the molybdenum effect in microstructure and mechanical properties, two microalloyed steels, with similar levels of niobium, titanium and vanadium, were prepared and forged. The steels have 0.056 and 0.160 percent by weight of molybdenum. The bars were homogenized, quenched and tempered. The specimens were machined for tensile and impact tests, hardness and microstructural analyzes by confocal and scanning electron microscopy. The ThermoCalc software was used to evaluate the carbides characteristics. The results show that steel with 0.160% Mo presents greater amount of bainite and austenite retained. It’s possible that formation of carbides by the secondary hardening mechanism increase in mechanical strength and a reduction in toughness.

Highlights

The oil and gas industry requires the development of steels that meet API 5L (American Petroleum Institute) requirements
The absorbed energy at impact was determined according to ASTM E23-16a standard using three specimens submitted to Charpy test at temperature of minus 20°C
The smaller grain in 056-Mo steel is possibly related to the higher niobium content

Summary

Introduction

The oil and gas industry requires the development of steels that meet API 5L (American Petroleum Institute) requirements. Forging is used in the manufacture of parts with complex geometry, such as valves for pipelines and connections for transporting gas and oil This process allows the manufacture of parts with a wide range of dimensions and shapes, microstructure control and an excellent combination of mechanical properties, after quenching and tempering treatments[3]. Associated with the forging process, the addition of niobium, titanium and vanadium at levels of up to 0.15% by weight promotes solid solution hardening and grain refinement These elements favor the formation of finely dispersed carbonitrides in the matrix during the austenite-ferrite transformation and are effective in increasing mechanical strength and tenacity[6,7]. A qualitative analysis and the carbide stoichiometry were determined through thermodynamic simulations with the ThermoCalc software

Experimental Procedures

Results and Discussion

Conclusions