# Determination of CCT Diagram by Dilatometry Analysis of High-Strength Low-Alloy S960MC Steel

^{1}

^{2}

^{*}

## Abstract

**:**

_{c1}and A

_{c3}increase with increasing heating rate. The A

_{c1}temperature increased by 54 °C and the A

_{c3}temperatures by 24 °C as the heating rate increased from 0.1 °C/s to 250 °C/s. The austenite decomposition temperatures have a decreasing trend in the cooling phase with increasing cooling rate. As the cooling rate changes from 0.03 °C/s to 100 °C/s, the initial transformation temperature drops from 813 °C to 465 °C. An increase in the cooling rate means a higher proportion of bainite and martensite. At the same time, the hardness increases from 119 HV10 to 362 HV10.

## 1. Introduction

_{8/5}), and the type of additional material are critical welding parameters affecting the final properties of HAZ. The heat-affected zone can be divided into four base regions: coarse-grained heat-affected zone (CGHAZ), fine grains heat-affected zone (FGHAZ), intercritical heat-affected zone (ICHAZ), and sub-critical heat-affected zone (SCHAZ).

_{C1}and a tempering soft zone when the temperature does not exceed A

_{C1}. These soft zones are typical for high-strength steels, with a lower hardness than the base material and a reduction in yield strength [15,16,17,18,19].

## 2. Materials and Methods

#### 2.1. Experimental Material

_{m}, yield strength—R

_{p0.2}, and percentage elongation after fracture—A).

#### 2.2. Preparation of Samples for Dilatometric Analysis

#### 2.3. Determination of the Dilatometric Curve

- The heating rate’s effect on shifts in the transformation temperatures, A
_{c1}and A_{c3}. For this experiment, eight variants with different heating rate values were useds from 0.1 °C/s to 250 °C/s. The cooling method was not considered. These variants are listed in Table 4 (aligning with increasing heating rate). Program control cycles are shown in Figure 3 (left). - The cooling rate affects the resulting austenite transformation temperatures in the cooling phase, microstructure, and hardness. We used eight variants with different cooling rates from 0.03 °C/s to 100 °C/s for this experiment. The heating method was not considered. These variants are listed in Table 5 (aligning with increasing cooling rate). Program control cycles are shown in Figure 3 (right). Subsequently, a CCT diagram was created from the data analysis.
- The effect of heating and cooling rates on the resulting grain size. All variants listed in Table 3 were used for this experiment. Out of these, variants have a constant heating rate but a different cooling rate. Variants with significantly different heating rates were used but with a constant cooling rate. We assessed which parameter (heating or cooling rate) would have a greater effect on the resulting grain size.

_{c1}and A

_{c3}, transformation temperatures of austenite and grain size). The influence of the input parameters must be verified at low levels of values, as well as at high ones. For this reason, the input parameters were chosen in a logarithmic series. A logarithmic function was used as a mathematical model to describe functional dependence. Furthermore, the Shapiro-Wilk test was used to confirm the normality of the regression residues.

#### 2.4. Methodology for Determining Phase Transition Temperatures from Dilatometry Curves

#### 2.5. Microstructure Observation and Hardness Measuring

## 3. Results and Discussion

#### 3.1. The Heating Rate’s Effect on Shifts in the Transformation Temperatures A_{c1} and A_{c3}

_{c1}and full austenitization temperature A

_{c3}were determined from eight dilatometry curves. We used both methods for determining temperature—the three tangent method and the first derivation of the dilatometry curve method.

_{c1}and A

_{c3}) at which the first derivative acquires zero.

_{c1}and A

_{c3}.

_{c1}temperature) increases with an increased heating rate. This trend was observed in other works [36,43]. The A

_{c3}temperature, when complete austenitization is achieved, was almost constant up to a heating rate of 10 °C/s. At heating rates higher than 10 °C/s, it began to grow. The differences between A

_{c3}and A

_{c1}decreased with an increased heating rate, as shown in Figure 8. The phenomena described are important in creating the width of the heat-affected zone and welding process modeling.

#### 3.2. Austenite Transformation Temperatures in Cooling Phase

_{2}and T

_{3}, occurred in addition to the first -T

_{1}and last -T

_{4}transformation temperatures and were also evaluated from the three tangent method. Curves with multiple transformation temperatures and optical microstructural analyses were important in determining all the excluded structural phases. The transformation temperatures are provided in Table 7.

_{C1}temperature. A slow cooling rate and high transformation temperature are typical signs of diffusion transformation of austenite to ferrite or perlite, particularly in the case of low-carbon and low-alloyed steel [4,6,36,44,45,46]. Austenite decomposition occurs at a lower temperature with an increasing cooling rate, indicating a bainite or martensite structure. This process occurred with the shear transformation of austenite [44]. Hardness measurements for all samples were performed to compare with the microstructure while considering hardness the ferrite–perlite steel structure, next bainite, martensite and their mixture [4,6,36,47]. The cooling rate’s effect on the values of the first and last austenite decomposition transformation temperature for both methods of determination is shown in Figure 9.

#### 3.3. The Analysis of the Hardness after Cooling

#### 3.4. The Microstructural Analysis and Design of CCT Diagram

#### 3.5. The Effect of Heating and Cooling Rates on the Resulting Grain Size

## 4. Conclusions

_{c1}and A

_{c3}was investigated. The influence of the cooling rate on the final microstructure properties was also investigated. The executed measurements and experiments led to the creation of a CCT diagram for structural steel S960MC with a 3 mm thickness and specific chemical composition stated in Table 1. The main conclusions of the above research are as follows:

- Both methods, the three tangent method and the first derivation of the dilatometry curve method, are suitable for determining transformation temperatures. The deviation of the first derivation method from the tangent method was a maximum of 2.1% (determination of transformation temperatures in the heating phase) and a maximum of 3.7% (determination of transformation temperatures in the cooling phase).
- The austenitic transformation (A
_{c1}temperature) increases with an increased heating rate. The dependence of these two parameters was tested by regression analysis. A linear regression model with a logarithmic function was used, which demonstrated the strong dependence of parameters (coefficient of determination was R^{2}= 0.9). This analysis shows that in the range of heating rates from 0.1 to 250 °C/s, the temperature A_{c1}increased from 749 °C to 804 °C. - The A
_{c3}temperature, when complete austenitization is achieved, was almost constant up to a heating rate of 5 °C/s (average value was 854 °C and standard deviation was 4 °C). At heating rates higher than 10 °C/s, it began to grow. From a heating rate of 10 °C/s to 250 °C/s the dependence was also described by a logarithmic function with coefficients of determination equal to R^{2}= 0.96. The phenomena described are important in creating the width of the heat-affected zone and welding process modeling. The welding processes with a high heating rate of the base material (laser welding and electron beam welding) will be characterized by a narrow HAZ. This eliminates the “softening effect of HAZ” that is typical for HSLA steel welding. - The dependence of the cooling rate on the transformation temperatures T1 and T4 of austenite decomposition is significant and has the opposite trend as in the heating phase. As the cooling rate increases, the austenite decomposition temperature decreases. A linear regression model with a logarithmic function was used, which demonstrated the strong dependence of parameters (coefficient of determination was R
^{2}= 0.98 for temperature T1 and R^{2}= 0.97 for temperature T4). The data are valid for the three tangent method. This analysis shows that in the range of cooling rates from 0.03 °C/s to 100 °C/s, the temperature T1 increased from 796 °C to 449 °C and temperature T4 increased from 734 °C to 394 °C. - A slow cooling rate and high transformation temperature are typical signs of diffusion transformation of austenite to ferrite or perlite. The microstructure of steel at a cooling rate of 1 °C/s and 3 °C/s comprises a mixture of ferrite and bainite (bainite predominates). Martensite appears in the microstructure only at a cooling rate of 10 °C/s. Its content increases with the increased cooling rate. At a cooling rate of 30 °C/s, its content is at the level of 91% and at a cooling rate of 100 °C/s the microstructure is fully martensitic.
- The hardness of the samples increased with the increased cooling rate due to the higher percentage of hardness phases such as bainite and martensite. The dependence of these two parameters was tested by regression analysis as well. A linear regression model with a logarithmic function was used. The coefficient of determination was R
^{2}= 0.97, which means a strong tightness of the variables and a well-chosen regression function. This analysis shows that in the range of cooling rates from 0.03 °C/s to 100 °C/s, the hardness increased from 116 HV10 to 362 HV10. The hardness of the base material 360HV10 will be achieved according to this model at a cooling rate of 94 °C/s. This cooling rate represents a time t_{8/5}of 3.6 s, which can be achieved by using concentrated heat source welding methods. - As the cooling rate increases, a trend of decreasing average grain size can be observed. However, regression analysis did not show strong parameter tightness using the full cooling rate range (from 0.03 °C/s to 100 °C/s). The coefficient of determination was R
^{2}= 0.72 for the logarithmic regression function and R^{2}= 0.85 for the power regression function. Data selection and analysis, however, demonstrated these findings. We can state that the cooling rate significantly affects grain size. Variants (H50 C3, H50 C10 and H50 C30) confirmed this finding when the heating rate was constant (50 °C/s), and grain size significantly decreased from 9.4 µm to 4.4 µm with the rising cooling rate (from 3 °C/s to 30 °C/s). This represents a decrease of 53% with a cooling rate change of 27 °C/s. The effect of the heating rate is not significant on the change in grain size. The cooling rate was constant for variants H50 C10 and H0.1 C10 (10 °C/s), and the heating rates were 0.1 and 50 °C/s at the same grain size (5.7 µm and 5.8 µm). The difference in grain size in these two variants is up to 2% with a change in heating rate of approximately 50 °C/s. It was also confirmed at the tests for the cooling rate of 30 °C/s and heating rates of 0.5 and 50 °C/s. The difference in grain size for both cases was <10%.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

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**Figure 1.**The martenzite–bainitic structure (

**left**) and EBSD grain size analysis (

**right**) of tested S960MC steel (HV 20 kV, step size 0.2 um).

**Figure 2.**The schematic shape of samples for the dilatometric test with red lines indicating the machined sides.

**Figure 3.**Different program temperature cycles for the control dilatometer DIL 805L;

**left**—heating phase;

**right**—cooling phase.

**Figure 4.**Dilatometry curves for 10 °C/s heating rate and 1 °C/s cooling rate (variant H10 C1) with three tangents applied.

**Figure 5.**The heating part of the dilatometry curve for a 10 °C/s heating rate with applied first derivative.

**Figure 6.**The cooling part of the dilatometry curve for a 1 °C/s cooling rate with the first derivative applied.

**Figure 9.**The influence of the cooling rate on transformation temperatures T

_{1}and T

_{4}:

**left**—three tangent method;

**right**—first derivation method.

**Figure 13.**SEM micrographs and EBSD analysis of grain size at different variants of the heating rate and cooling rates.

According | Chemical Composition wt.%—Strenx 960MC | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|

C | Si | Mn | P | S | Al | Nb | V | Ti | Mo | B | |

EN 10149-2 * | 0.200 | 0.60 | 2.200 | 0.025 | 0.010 | 0.015 | 0.090 | 0.200 | 0.250 | 1.000 | 0.005 |

Material certificate ** | 0.085 | 0.180 | 1.060 | 0.010 | 0.003 | 0.036 | 0.002 | 0.007 | 0.026 | 0.109 | 0.001 |

Experimental measurement average value/standard deviation | 0.055/ 0.0024 | 0.168/ 0.0045 | 1.203/ 0.0047 | <0.010 | <0.010 | 0.037/ 0.00023 | <0.005 | 0.005/ 0.00025 | 0.023/ 0.00017 | 0.086/ 0.00012 | - |

Cu | Cr | Ni | N | CEV | CET | ||||||

EN 10149-2 * | - | - | - | - | - | - | |||||

Material certificate ** | 0.010 | 1.080 | 0.070 | 0.005 | 0.506 | 0.258 | |||||

Experimental measurement average value/standard deviation | <0.005 | 1.056/ 0.0081 | 0.046/ 0.0017 | - | 0.489 | 0.238 |

According | Angle of Rolling Direction | Mechanical Properties S960MC, Thickness 3 mm | ||||||
---|---|---|---|---|---|---|---|---|

R_{p0.2} [MPa] | R_{m} [MPa] | R_{p0.2}/R_{m} | A [%] | |||||

EN 10149-2 | - | min. 960 | 980–1250 | - | min. 7 | |||

Average R_{p0.2} [Mpa]/Standard deviation [Mpa] | Average R_{m} [Mpa]/Standard deviation [Mpa] | Average R_{p0.2}/Average R_{m} | Average A [%]/Standard deviation [%] | Coefficient of area anisotropy P * [%] | ||||

PR_{m} | PR_{p0.2} | PA | ||||||

Experimental measurement | 0° | 1007/15.6 | 1092/7.3 | 0.92 | 7.9/0.28 | - | - | - |

45° | 1018/5.1 | 1106/7.4 | 0.92 | 6.7/0.16 | 1.2 | 1.1 | −14.4 | |

90° | 1044/7.3 | 1124/4.6 | 0.93 | 6.5/0.05 | 2.9 | 3.6 | −17.0 |

_{p0.2}, R

_{m}and A.

Variant Designation | H5 C0.03 | H100 C0.1 | H1 C0.3 | H10 C1 | H50 C3 | H0.1 C10 | H50 C10 |

Heating rate °C/s | 5 | 100 | 1 | 10 | 50 | 0.1 | 50 |

Cooling rate °C/s | 0.03 | 0.1 | 0.3 | 1 | 3 | 10 | 10 |

Variant Designation | H0.5 C30 | H50 C30 | H250 C100 | H250 C100 * | H50 C200 | H50 C200 * | |

Heating rate °C/s | 0.5 | 50 | 250 | 250 * | 50 | 50 * | |

Cooling rate °C/s | 30 | 30 | 100 | 100 * | 200 | 200 * |

**Table 4.**The chosen variants for analyzing the shift in transformation temperatures A

_{c1}and A

_{c3}.

Heating Rate °C/s | 0.1 | 0.5 | 1 | 5 | 10 | 50 | 100 | 250 |
---|---|---|---|---|---|---|---|---|

Cooling rate °C/s | 10 | 30 | 0.3 | 0.03 | 1 | 3 | 0.1 | 100 |

Variant designation | H0.1 C10 | H0.5 C30 | H1 C0.3 | H5 C0.03 | H10 C1 | H50 C3 | H100 C0.1 | H250 C100 |

**Table 5.**The chosen variants for analyzing austenite transformation temperatures in the cooling phase, final microstructure, and hardness.

Cooling Rate °C/s | 0.03 | 0.1 | 0.3 | 1 | 3 | 10 | 30 | 100 |
---|---|---|---|---|---|---|---|---|

Heating rate °C/s | 5 | 100 | 1 | 10 | 50 | 0.1 | 0.5 | 250 |

Variant | H5C 0.03 | H100 C0.1 | H1 C0.3 | H10 C1 | H50 C3 | H0.1 C10 | H0.5 C30 | H250 C100 |

**Table 6.**The effect of the heating rate on shifts in the transformation temperatures A

_{c1}and A

_{c3}.

Heating Rate [°C/s] | 0.1 | 0.5 | 1 | 5 | 10 | 50 | 100 | 250 | |
---|---|---|---|---|---|---|---|---|---|

Variant Designation | H0.1 C10 | H0.5 C30 | H1 C0.3 | H5 C0.03 | H10 C1 | H50 C3 | H100 C0.1 | H250 C100 | |

Three tangent method | A_{c1} [°C] | 758 | 761 | 763 | 770 | 774 | 795 | 795 | 812 |

A_{c3} [°C] | 858 | 858 | 850 | 850 | 853 | 863 | 870 | 882 | |

First derivation method | A_{c1} [°C] | 753 | 758 | 763 | 770 | 788 | 806 | 812 | 832 |

A_{c3} [°C] | 855 | 850 | 850 | 850 | 870 | 880 | 886 | 898 | |

Difference between used methods | diff. A_{c1} [%] | 0.66 | 0.40 | 0.00 | 0.00 | 1.78 | 1.36 | 2.09 | 2.40 |

diff. A_{c3} [%] | 0.35 | 0.94 | 0.00 | 0.00 | 1.95 | 1.93 | 1.81 | 1.78 |

Cooling Rate [°C/s] | 0.03 | 0.1 | 0.3 | 1 | 3 | 10 | 30 | 100 | |
---|---|---|---|---|---|---|---|---|---|

Variant Designation | H5C 0.03 | H100 C0.1 | H1 C0.3 | H10 C1 | H50 C3 | H0.1 C10 | H0.5 C30 | H250 C100 | |

Three tangent method | T_{1} [°C] | 813 | 752 | 690 | 620 | 594 | 553 | 497 | 465 |

T_{2} [°C] | - | 744 | - | - | - | 534 | 465 | 456 | |

T_{3} [°C] | - | - | - | - | - | 514 | 469 | 442 | |

T_{4} [°C] | 750 | 704 | 612 | 580 | 539 | 469 | 431 | 432 | |

First derivation method | T_{1} [°C] | 784 | 744 | 679 | 626 | 602 | 551 | 491 | 463 |

T_{4} [°C] | 749 | 704 | 612 | 571 | 532 | 468 | 429 | 426 | |

Difference between used methods | diff. T_{1} [%] | 3.70 | 1.08 | 1.62 | 0.96 | 1.33 | 0.36 | 1.22 | 0.43 |

diff. T_{4} [%] | 0.13 | 0.00 | 0.00 | 1.58 | 1.32 | 0.21 | 0.47 | 1.41 |

Cooling Rate [°C/s] | Variant | Hardness HV10 | Average Hardness HV10/Standard Deviation HV10 | |||||
---|---|---|---|---|---|---|---|---|

0.03 | H5C 0.03 | 118 | 116 | 122 | 117 | 119 | 123 | 119/2.54 |

0.1 | H100 C0.1 | 152 | 145 | 157 | 144 | 148 | 143 | 148/4.94 |

0.3 | H1 C0.3 | 196 | 201 | 203 | 197 | 200 | 199 | 199/2.36 |

1 | H10 C1 | 223 | 231 | 227 | 227 | 227 | 223 | 226/2.75 |

3 | H50 C3 | 236 | 238 | 240 | 232 | 237 | 239 | 237/2.58 |

10 | H0.1 C10 | 276 | 268 | 279 | 281 | 273 | 271 | 275/4.50 |

30 | H0.5 C30 | 349 | 344 | 352 | 352 | 346 | 342 | 348/3.81 |

100 | H250 C100 | 357 | 365 | 356 | 372 | 361 | 360 | 362/5.40 |

Base material | 362 | 363 | 357 | 362 | 360 | 358 | 360/2.21 |

**Table 9.**The chosen variants for analyzing the final microstructure, hardness, and austenite transformation temperatures in the cooling phase.

Variant Designation | Cooling Rate [°C/s] | Grain Size ECD ** [µm] Area 500 × 500 µm, Step 0.2 µm, Average Value/Standard Deviation |
---|---|---|

H5 C0.03 | 0.03 | 26.2/15.94 |

H100 C0.1 | 0.1 | 15.0/8.87 |

H1 C0.3 | 0.3 | 10.0/7.89 |

H10 C1 | 1 | 9.0/6.98 |

H50 C3 | 3 | 9.4/6.01 |

H50 C10 | 10 | 5.7/4.22 |

H0.1 C10 | 10 | 5.8/4.71 |

H50 C30 | 30 | 4.4/2.71 |

H0.5 C30 | 30 | 4.9/3.17 |

H250 C100 | 100 | 5.3/3.76 |

H250 C100 * | 100 * | 4.9/2.89 |

H50 C200 | 200 | 5.8/4.64 |

H50 C200 * | 200 * | 5.1/3.19 |

As rolled | - | 4.1/2.34 |

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**MDPI and ACS Style**

Moravec, J.; Mičian, M.; Málek, M.; Švec, M.
Determination of CCT Diagram by Dilatometry Analysis of High-Strength Low-Alloy S960MC Steel. *Materials* **2022**, *15*, 4637.
https://doi.org/10.3390/ma15134637

**AMA Style**

Moravec J, Mičian M, Málek M, Švec M.
Determination of CCT Diagram by Dilatometry Analysis of High-Strength Low-Alloy S960MC Steel. *Materials*. 2022; 15(13):4637.
https://doi.org/10.3390/ma15134637

**Chicago/Turabian Style**

Moravec, Jaromír, Miloš Mičian, Miloslav Málek, and Martin Švec.
2022. "Determination of CCT Diagram by Dilatometry Analysis of High-Strength Low-Alloy S960MC Steel" *Materials* 15, no. 13: 4637.
https://doi.org/10.3390/ma15134637