Investigation of Hardenability in Low-Carbon Alloy Steels Using Jominy End-Quenched Test
Summary
This study evaluates the hardenability of two low-carbon alloy steel samples (P1 and P6) using the Jominy end-quenched test. The experiment focuses on analyzing the effect of chemical compositions and microstructural differences between the base alloy (P1) and an alloy with Cr-Mo-V additives (P6). Results indicate a significantly higher hardenability in P6 due to carbide precipitation and martensitic transformation.
Introduction
Background Knowledge
Hardenability, or the ability of steel to form martensite during heat treatment, is a critical property for engineering applications requiring specific mechanical properties, such as strength and wear resistance. Predicting and optimizing hardenability allows for tailored designs of steel components used in aerospace, automotive, and construction industries. Traditional methods to evaluate hardenability, like generating Continuous Cooling Transformation (CCT) and Time-Temperature Transformation (TTT) diagrams, are both time-consuming and costly. The Jominy end-quenched test provides a practical and efficient alternative for evaluating hardenability.
Limitations of Current Knowledge
Although alloying elements such as chromium (Cr), molybdenum (Mo), and vanadium (V) are known to enhance hardenability, their specific effects on microstructure and hardness profiles need further clarification in various low-carbon alloy systems. Additionally, discrepancies in quenching methods during practical applications, such as quenching media and conditions, can affect experimental results and limit the accuracy of predicting real-world hardenability.
Fig 1. Example TTT diagram of 52100 steel that shows the limit to produce the martensite by
lowing the quenching temperature
Literature Connection
The study builds on established theories of phase transformation and hardenability:
1.
Martensitic Transformation: The formation of martensite occurs through a diffusionless transformation during rapid cooling, leading to a body-centered tetragonal (BCT) structure with high strength. The extent of martensite formation is determined by cooling rates, prior austenite grain size, and alloy content.
2.
Role of Alloying Elements: Elements like Cr, Mo, and V are carbide formers that influence phase transformation by stabilizing austenite, delaying pearlite formation, and promoting martensite formation.
3.
Practical Implications: By leveraging simplified methods like the Jominy end-quenched test, researchers can estimate the hardenability of alloy steels more efficiently while maintaining alignment with ASTM standards.
This background highlights the necessity of using the Jominy end-quenched test as an accessible and reliable method to analyze the influence of alloying elements on hardenability while addressing the practical challenges of quenching conditions.
Research Aim and Objectives
Aim
To evaluate and compare the hardenability of P1 (base alloy) and P6 (base-Cr-Mo-V alloy) through the Jominy end-quenched test and to investigate the relationship between their hardenability, microstructural transformations, and chemical compositions.
Objectives
1.
Conduct Jominy End-Quenched Tests: Perform ASTM A255-based Jominy end-quenched tests on P1 and P6 samples to determine their hardness profiles along the quenched length, focusing on the formation of martensite and other microstructures at varying distances.
2.
Microstructural Analysis: Examine the quenched, center, and air-cooled regions of the samples using optical microscopy (at 200x magnification) to identify key microstructures such as lath martensite, bainite, ferrite, and pearlite.
3.
Hardness Measurement and Profile Comparison: Measure Vickers hardness (Hv10) at specific intervals from the quenched end of each sample and analyze the differences in hardness profiles to identify the Jominy distance corresponding to 50% martensite formation.
4.
Correlation with Alloy Composition: Relate the observed differences in hardenability and microstructure to the chemical compositions of P1 and P6, particularly focusing on the role of Cr, Mo, and V as carbide formers and their effects on the delay of austenite-ferrite phase transformation and martensite stabilization.
Methodology
1. Sample Preparation
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Two alloy steel samples, P1 (base) and P6 (Cr-Mo-V additive alloy), were prepared from commercial-grade S690 steel with chemical compositions as outlined in ASTM A255.
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Steel slabs were hot-rolled at 900°C to a plate thickness of 35 mm, air-cooled, and sectioned into 60 mm × 10 mm × 10 mm specimens.
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Samples were heated in a furnace at 950°C to ensure homogeneity before quenching.
Tab 2. Chemical Composition (wt%) of the sample P1 and P6 which P1 is the based material of the sample and P6 is the Cr-Mo-V additive alloy
2. Experiment
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Quenching Procedure:
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The heated specimens were immediately removed from the furnace and one end was quenched in still water to a depth of 3 mm, following a simplified ASTM A255 Jominy test procedure.
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The other end was left to air-cool, simulating varied cooling conditions along the specimen length.
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Surface Preparation:
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Post-quenching, specimens were sectioned longitudinally, and the surfaces were ground sequentially with 400, 800, and 1200 grit abrasive papers.
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Markings were made at 3 mm intervals from the quenched end to the air-cooled end to standardize hardness measurements.
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Hardness Testing:
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Vickers hardness (Hv10) was measured at each marked position using a Vickers hardness tester. Indentations were placed at consistent distances, minimizing overlap to prevent measurement interference.
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Microstructure Observation:
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Micrographs of the quenched end, mid-section, and air-cooled end were captured using an optical microscope at ×200 magnification to analyze the phase distributions.
3. Data Analysis
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Hardness Analysis:
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Vickers hardness values were plotted against distance from the quenched end to compare the hardenability of P1 and P6.
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The Jominy distance corresponding to 50% martensite formation was identified for both samples.
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Microstructural Analysis:
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Micrographs were analyzed to identify and quantify microstructures, including lath martensite, bainite, ferrite, and pearlite, at critical regions along the quenched length.
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Observations were correlated with hardness profiles and chemical compositions to determine the effects of alloying elements on phase transformations.
Results
Key Findings
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The Vickers hardness measurements revealed that P6 consistently exhibited higher hardness values across the quenched length compared to P1, with a maximum hardness of 519.2 Hv at the quenched end for P6 and 447 Hv for P1.
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The hardenability of P6 was significantly greater than that of P1, as indicated by the longer Jominy distance corresponding to 50% martensite formation: 33 mm for P6 versus 27 mm for P1. This suggests that the Cr-Mo-V additives in P6 extended the effective hardening zone.
Tab 3. Jominy end-quenched test result of P1 and P6 corresponds to Jominy distance that first 3mm is the quenched end surface by still water.
Comparative Analysis
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Alloying Effects:
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The superior hardenability of P6 was primarily attributed to the Cr-Mo-V alloying elements, which acted as strong carbide formers. These elements promoted the formation of intermetallic compounds and stabilized the martensitic transformation during rapid cooling by trapping carbon atoms within the microstructure.
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In P6, the presence of alloy carbides reduced the diffusivity of carbon, enhancing the stability of martensite over a broader cooling range.
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Microstructural Observations:
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Optical micrographs at the quenched end of P6 revealed a dominant presence of lath martensite, with minor amounts of bainite, contributing to its higher hardness.
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P1, in contrast, displayed a more heterogeneous microstructure. While lath martensite and bainite were visible at the quenched end, the proportion of martensite decreased more rapidly along the sample length.
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At the air-cooled end:
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P6 retained small amounts of bainite, indicating slower phase transformation due to the Cr-Mo-V additives.
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P1 exhibited a noticeable increase in ferrite and pearlite, confirming its lower hardenability and reduced capacity to retain martensitic phases under slower cooling conditions.
Fig 2. Jominy end-quenched test result graphed by Vickers hardness and distance from the quenched end that the significant increase of hardness in P6 between the Jominy distance of 20mm to 30mm.
Figure 3: Micrograph of the P1 and P6 sample at x200 with the scale bar of 100um and 50um. (First column: P1 and second column: P6)
Discussion
Comparison with Background Knowledge
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The experimental results align with the established role of alloying elements in enhancing hardenability. As highlighted in the background, elements like Cr, Mo, and V are strong carbide formers that stabilize martensite by forming intermetallic compounds with carbon. This effect was evident in P6, where these additives contributed to higher hardness values and extended martensite formation, as predicted by TTT and CCT theories.
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The microstructural observations confirmed the theoretical understanding of phase transformation dynamics. P6 displayed a higher proportion of martensite and bainite compared to P1, correlating with the background discussion on delayed ferrite and pearlite formation due to alloying elements.
Figure 4: Carbides former in steel with electron number and atomic radius that contribute to forming carbides
Key Findings in Context
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The measured hardness values (519.2 Hv at the quenched end for P6 versus 447 Hv for P1) and the extended Jominy distance for 50% martensite formation (33 mm for P6 versus 27 mm for P1) directly support the theoretical influence of Cr-Mo-V in increasing hardenability. These findings underscore the effectiveness of carbide formers in delaying phase transformation and promoting martensitic stability during rapid cooling.
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The microstructural transition along the quenched length—from martensite dominance at the quenched end to ferrite and pearlite at the air-cooled end—provides further validation of the chemical composition's impact on phase transformations under different cooling rates.
Limitations of the Experiment
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Quenching Method: The simplified quenching process, involving immersion in still water rather than a standard water jet, introduced a higher quenching temperature at the quenched end. This may have reduced the cooling rate, potentially affecting the formation of martensite and leading to discrepancies in hardness measurements, particularly the anomalous inflection observed between 20 mm and 25 mm.
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Measurement Constraints: The hardness measurements were limited to intervals along the sample length, which may have omitted finer variations in hardness and microstructural transitions. Additionally, variations in Vickers indentation spacing could introduce localized measurement errors.
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Microstructure Resolution: While optical microscopy provided insights into the phase distributions, it lacked the resolution to analyze finer carbide precipitates or quantify their precise distribution, which could further explain the differences in hardenability between P1 and P6.
Figure 5: Converting from Jominy end-quenched test result to CCT diagram that can find the specific phase transformation following CCT diagram.
Conclusion
Final Remark
The study successfully evaluated the hardenability differences between P1 (base alloy) and P6 (Cr-Mo-V additive alloy) using the Jominy end-quenched test and microstructural analysis. The results demonstrated that P6 exhibited significantly higher hardness and hardenability compared to P1, with a longer 50% martensite Jominy distance (33 mm for P6 vs. 27 mm for P1). This enhanced hardenability in P6 was attributed to the Cr-Mo-V additives, which stabilized martensite formation and delayed the onset of ferrite and pearlite phases, as evidenced by both Vickers hardness profiles and microstructural observations.
Future Research
1.
Advanced Microstructural Analysis
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Utilizing high-resolution imaging techniques such as Scanning Electron Microscopy (SEM) or Transmission Electron Microscopy (TEM) can provide a deeper understanding of the distribution, morphology, and composition of carbide precipitates. This improvement is highly meaningful as it allows precise quantification of the role of carbide-forming elements (e.g., Cr, Mo, V) in enhancing hardenability, bridging the gap between theoretical predictions and experimental observations. This approach could yield insights into optimizing alloy designs for specific mechanical properties.
2.
Thermodynamic Modelling
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Incorporating thermodynamic simulation tools like Thermo-Calc to predict phase transformations and cooling curves offers a robust method to complement experimental results. This approach can provide a theoretical framework to anticipate microstructural changes based on alloy composition and cooling rates, minimizing the need for costly and time-intensive experimental trials. By aligning predicted and observed results, this method can improve the accuracy of hardenability assessments and guide the design of future experiments or industrial processes.
These two improvements not only address the critical aspects of microstructural understanding and predictive modeling but also have practical implications for optimizing alloy design and heat treatment processes in industrial applications.
Reference
Reference list