Effect of microstructures with the same chemical composition and similar hardness levels on tribological behavior of a low alloy steel

Abstract Copper-containing low alloy steel based on ASTM A707 5L grade is widely used for structural parts of offshore wells. Applications of the steel for Ultra-deepwater development require excellent low temperature toughness from the viewpoint of marine accident prevention. However it is difficult to stably obtain good weld joint toughness because the welding condition is inevitably scattering. With those backgrounds, this paper focuses on metallurgical factors controlling the HAZ toughness of A707 modified steel. Potential factors considered are the grain size, M-A and precipitates. A challenge is demonstrated to improve the HAZ toughness by optimizing the Cu and Mn contents. In this study, we investigated mechanical properties including crack tip opening displacement (CTOD) and we observed microstructure using welding tests or various weld heat cycle specimens. The weld heat affected zone (HAZ) of a conventional material had good toughness for the low heat input condition. However it was remarkably decreased for the high heat input condition due to the precipitating martensite-austenite constituent (M-A) in local brittle zones (LBZ). The weld test results indicated the importance of suppressing the formation of M-A in order to improve toughness in the HAZ of the steel. Thereby, we challenged the optimization of chemical composition for HAZ toughness improvement. Cu had no bad influence on the HAZ toughness. It was demonstrated that the HAZ toughness is recovered by good use of Cu precipitates in SC cycle. Moreover the area fraction of M-A is decreased in keeping with Mn content, which leads to the improvement of the ICCG HAZ toughness. Based on our study, the recommended amounts of Cu and Mn are more than 1.0 mass% and less than 0.6 mass%, respectively, to ensure the HAZ toughness, especially ICCG HAZ toughness.

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Effect of Molybdenum on the Impact Toughness of Heat-Affected Zone in High-Strength Low-Alloy Steel

Materials ◽

10.3390/ma14061430 ◽

2021 ◽

Vol 14 (6) ◽

pp. 1430

Author(s):

Xiaoyan Wu ◽

Pengcheng Xiao ◽

Shujing Wu ◽

Chunliang Yan ◽

Xuegang Ma ◽

...

Keyword(s):

Chemical Composition ◽

Alloy Steel ◽

Heat Affected Zone ◽

Laser Scanning ◽

Lattice Misfit ◽

High Strength ◽

Optical Microscope ◽

Low Alloy Steel ◽

Number Density ◽

The Impact

The microstructure, precipitates, and austenite grain in high-strength low-alloy steel were characterized by optical microscope, transmission electron microscope, and laser scanning confocal microscopy to investigate the effect of Mo on the toughness of steel. The microstructure was refined and the toughness was enhanced after the addition of 0.07% Mo in steel. The addition of Mo can suppress the Widmanstätten ferrite (WF) formation and promote the transformation of acicular ferrite (AF), leading to the fine transformed products in the heat-affected zone (HAZ). The chemical composition of precipitates changed from Nb(C, N) to (Nb, Mo)(C, N) because of the addition of Mo. The calculated lattice misfit between Nb(C, N) and ferrite was approximately 11.39%, while it was reduced to 5.40% for (Nb, Mo)(C, N), which significantly affected the size and number density of precipitates. A detailed analysis of the precipitates focusing on the chemical composition, size, and number density has been undertaken to understand the contribution of Mo on the improvement of steel toughness.

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Improving the Toughness of the Weld-Heat-Affected Zone of Cu-Containing Low-Alloy Steel for Offshore Applications by Optimizing Chemical Composition

Journal of Engineering Materials and Technology ◽

10.1115/1.4047964 ◽

2020 ◽

Vol 143 (1) ◽

Author(s):

Yuta Honma ◽

Gen Sasaki ◽

Kunihiko Hashi ◽

Fumiyoshi Minami

Keyword(s):

Chemical Composition ◽

Alloy Steel ◽

Heat Affected Zone ◽

Negative Influence ◽

Area Fraction ◽

Test Results ◽

Low Alloy Steel ◽

Potential Factors ◽

Structural Parts ◽

Haz Toughness

Abstract Copper-containing low-alloy steel based on the ASTM A707 5L grade is widely used for structural parts of offshore wells. However, it is difficult to stably obtain good weld joint toughness. With this background, this paper focuses on the metallurgical factors controlling the heat-affected zone (HAZ) toughness of A707 modified steel. Potential factors considered are the grain size, the martensite–austenite constituent (M-A), and precipitates. Thus, the purpose was to clarify the effect of M-A and precipitates on HAZ toughness. Furthermore, Cu, Si, and Mn contents, which affect M-A and precipitates generations, were focused on and tried to improve HAZ toughness by optimizing their contents in ASTM A707 steel. The weld test results showed that the toughness of an intercritically coarsened grain HAZ (ICRCGHAZ) was remarkably lower than that of the other heat cycle pattern due to the formation of M-A. It is, therefore, essentially important to suppress the formation of M-A in order to improve toughness in the HAZ of the steel. Therefore, the chemical composition was optimized in an effort to improve HAZ toughness. Copper had no negative influence on the HAZ toughness. It was found that when the Mn and Si contents of the steel decreased, the area fraction of M-A decreased. Consequently, the ICRCG HAZ toughness is improved because the toughness increases with the decrease in the area fraction of M-A. The recommended amounts of Cu, Mn, and Si to ensure HAZ toughness are more than 1.0 wt%, less than 0.6 wt%, and less than 0.1 wt%, respectively.

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Mathematical model of forecast of strength of low carbon steel

Physical Metallurgy and Heat Treatment of Metals ◽

10.30838/j.pmhtm.2413.230321.49.734 ◽

2021 ◽

pp. 49-58

Author(s):

D.S. Kotenko

Keyword(s):

Mechanical Properties ◽

Regression Analysis ◽

Chemical Composition ◽

Yield Strength ◽

Alloy Steel ◽

The State ◽

Structural Elements ◽

Low Carbon ◽

Low Alloy Steel ◽

Short Period

Introduction. The use of different mathematical approaches to assessing and forecasting the quality characteristics of materials for different purposes is always relevant. The urgency of solving problems and problems of modern materials science with the use of methods of mathematical modeling allows to optimize technological processes of production, to determine in a short period of time the set parameters with minimal time and material costs. In the work using the method of regression analysis, the strength criteria of low-carbon low-alloy steel depending on the characteristics of the structure were evaluated. Materials and methods. Samples of Ст3пс steel grade made of a circle with a diameter of 24 mm were selected as the material for the study. The structure and mechanical properties were investigated at three reference points: at a distance of 0 mm from the center of the sample, 6 mm from the center of the sample and 12 mm from the center of the sample. The steel was investigated in the state of factory delivery, and after two modes of heat treatment to obtain ferritic-perlite and bainite structure. The following properties were determined: microhardness, tensile strength and yield strength, hardness and toughness at room temperature. The results of the experiment. Models for estimating mechanical properties were obtained using regression analysis. Models describing the relationship between the microhardness of pearlite and its area (R2 = 0.8366) in the state of factory delivery have a relatively high correlation coefficient; the score and the ultimate strength (R2 = 1.0) and yield strength (R2 = 0.8669) of steel after cooling in an oil medium; hardness and area of pearlite after hardening steel in the pearlite region (R2 = 0.7215). Conclusions. The practical significance of the work performed is the ability to perform a rapid analysis of the properties of rolled metal from steel Ст3пс based on determining the area of the structural elements and their scoring. However, it should be noted that the existing discrepancy between the results of the experiment and the forecast using the obtained models may be due to the influence of other factors. Such factors include the influence of chemical composition, incompleteness of formal axiomatics, which occurs when estimating the geometry of complex structural elements. Keywords: low-alloy steel; structure; chemical composition; mechanical properties; regression model; properties forecast

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Assessment of the Low Alloy Cast Steel Inoculation Effects with Chosen Additives

Archives of Foundry Engineering ◽

10.2478/v10266-012-0063-4 ◽

2012 ◽

Vol 12 (2) ◽

pp. 211-214 ◽

Cited By ~ 1

Author(s):

D. Bartocha ◽

C. Baron ◽

J. Suchoń ◽

J. Kilarski ◽

J. Szajnar ◽

...

Keyword(s):

Mechanical Properties ◽

Chemical Composition ◽

Surface Area ◽

Alloy Steel ◽

Cast Steel ◽

Solidification Process ◽

Low Alloy Steel ◽

Axial Zone ◽

Inoculation Treatment ◽

Alloy Cast

Assessment of the Low Alloy Cast Steel Inoculation Effects with Chosen Additives Structure, and thus the mechanical properties of steel are primarily a function of chemical composition and the solidification process which can be influenced by the application of the inoculation treatment. This effect depends on the modifier used. The article presents the results of studies designed to assess the effects of structural low alloy steel inoculation by selected modifying additives. The study was performed on nine casts modeled with different inoculants, assessment of the procedure impact was based on the macrostructure of made castings. The ratio of surface area equivalent to the axial zone of the crystals and columnar crystals zone was adopted as a measure of the inoculation effect.

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