Continuous cooling transformation diagram and mechanical properties in weld coarse-grain heat-affected zone of API X70 steel

Oct 29, 2026

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The work investigates continuous cooling transformation (CCT) in the coarse-grained heat-affected zone (CGHAZ) and simulates Charpy-sized impact specimens using a Gleeble 3800 thermomechanical simulator. Cooling rate strongly controls microstructure and low-temperature (−20 °C) Charpy impact toughness. Rising cooling rates reduce Ar3 and Ar1 transformation temperatures and impact toughness, while increasing hardness and bainite fraction. Slow cooling yields quasi-polygonal ferrite and pearlite; medium cooling gives bainite and quasi-polygonal ferrite; fast cooling produces fully bainitic microstructures confirmed by hardness measurements. Charpy toughness increases as cooling rate decreases due to softer ferrite.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages10

Reading Time25 min

AccessFree to download

Abstract

Introduction

Weldability and pipeline requirements

Heat-affected zone sub-zones and CGHAZ weakness

Cooling rate effects and transformation behavior

Sådhanå (2021)46:88 [https://doi.org/10.1007/s12046-021-01623-2](https://doi.org/10.1007/s12046-021-01623-2) 􀀃 Indian Academy of Sciences Continuous cooling transformation diagram and mechanical properties in weld coarse-grain heat-affected zone of API X70 steel AJIT KUMAR NAIK 1, * , RAKESH ROSHAN 1, KANWER SINGH ARORA2, NIKHIL SHAJAN2 and SUBASH CHANDRA MISHRA 1 1 Department of Metallurgical and Materials Engineering, National Institute of Technology, Rourkela 769 008, India 2 R&D, Tata Steel Limited, Jamshedpur 831 001, India e-mail: [ajitkumarnaik16@gmail.com](ajitkumarnaik16@gmail.com) MS received 20 September 2020; revised 23 March 2021; accepted 24 March 2021 Abstract. In the present work, continuous cooling transformation (CCT) of coarse-grained heat-affected zone (CGHAZ) and simulation of Charpy-sized impact specimens were performed using a Gleeble 3800 thermomechanical simulator. Results obtained from the dilation studies show signiﬁcant effect of cooling rates on microstructure and low-temperature (–20 􀀂 C) Charpy impact toughness. Phase transformation temperatures (Ar3 andAr1) and impact toughness decreased while hardness and amount of bainite increased with increasing cooling rates. At slow cooling condition (\ 5 􀀂 Cs–1) quasi-polygonal ferrite and pearlite phases were observed in the microstructure. At medium cooling rate (5–25 􀀂 Cs–1), bainite and quasi-polygonal ferrite were obtained in the microstructure. For still faster cooling rates, microstructure was completely bainitic in nature. The microstructures were conﬁrmed by hardness measurement where the hardness value for lower, medium and high cooling rates were 191–196, 213–214 and 234–253 HV, respectively. Charpy impact toughness increased with decrease in cooling rate due to the presence of softer ferrite phase. Keywords. Gleeble; CCT; CGHAZ; dilation; impact; toughness; API X70 . 1. Introduction To ensure adequate safety in long distance transportation of highly ﬂammable pressurized natural gas and crude oil, pipeline steels are preferred due to their high strength, good weldability, impact toughness and better strength to weight ratio [1] . Additionally, to ensure effective high-pressure transportation, large diameters pipelines are fabricated using thicker steels. API X70, a high-strength low-alloy steel processed through thermomechanical controlled rolling, is being extensively used in new gas transportation projects [2] . As pipelines go through cold regions and deep oceans, good negative temperature toughness is required to ensure safe use of pipeline steel. Hence, formation of soft ferrite phase should be promoted whereas formation of martensite and bainite should be minimized in order to improve toughness [3] . Fusion welding is the most common method for joining of such thicker sections. Heat-affected zone (HAZ) is the region adjacent to weld joints wherein the tailored microstructure of parent Ajit Kumar Naik and Rakesh Roshan equally contributed to this article. *For correspondence material, obtained through thermomechanical controlled processing (TMCP), is altered by the welding thermal cycle [4] . HAZ is divided into four different sub-zones on the basis of peak temperature experienced. The region close to fusion line where temperature goes above 1250 􀀂 C and austenite grain growth occurs is termed as coarse-grained heat-affected zone (CGHAZ) . The zone next to CGHAZ where temperature goes above 1050 􀀂 C and ﬁne grains are obtained is termed as ﬁne-grain heat-affected zone (FGHAZ) . The subzone of HAZ adjacent to FGHAZ where temperature goes above 750 􀀂 C is termed as intercritical heat-affected zone (ICHAZ) and the last farthest zone is called subcritical heat-affected zone (SCHAZ) [5–7] . Due to high peak temperature, precipitate dissolution and grain coarsening occurs in the CGHAZ [8] . This results in reduced prior austenite grain boundary area, which suppresses the nucleation of ferrite and pearlite bu

FAQs

What experimental and simulation method is used to study the CGHAZ in API X70 steel?

A Gleeble 3800 thermomechanical simulator is used to perform continuous cooling transformation experiments and to simulate Charpy-sized impact specimens.

How does increasing cooling rate affect the microstructure and mechanical properties?

Higher cooling rates lower transformation temperatures (Ar3 and Ar1) and reduce impact toughness, while hardness increases and the bainite fraction becomes larger.

What microstructures form under slow, medium, and high cooling conditions?

Slow cooling shows quasi-polygonal ferrite and pearlite; medium cooling produces bainite and quasi-polygonal ferrite; the fastest cooling results in a fully bainitic microstructure.

Oct 29, 2026

Upload by Rhys