**2.2. Influence of austenitizing parameters on the size of prior austenite grain**

Influence of austenitization parameters on the prior austenite grain size has been described in a quantitative way using chosen stereological and statistical parameters, such as: mean diameter and mean area of grain, and also the coefficient of variation of grain size *ν* was calculated. The *ν* coefficient is characterized by the inhomogeneity of grain sizes: the more heterogeneous grains in terms of size within the casting, the higher the values of variation coefficient. The tests were run for the austenitizing temperature range of - 910 970 oC with the "measurement step" – 15 oC and times of holding at the austenitizing temperature: 3 and 5 hours.

The character of austenite grain distributions was determined using the λ - Kolmogorov test of goodness of fit with normal distribution for logarithmed values (Fig. 8). The assumed significance level was = 0.01, with its limiting statistics value amounting to 1.63. Selected logarithm-normal layouts of mean diameters and mean surface areas of former austenite grains for austenitization option of 925 oC and holding time 3 hours, are shown in Fig. 8. Obtained results of the tests are presented in Table 3 and 4 and graphically shown in Fig. 9 ÷ 11.


**Table 3.** The results of measurements and calculations of the size of prior austenite grains for the cast steel


Regenerative Heat Treatment of Low Alloy Cast Steel 29

**Figure 9.** Influence of austenitization temperature on: a) mean grain diameter; b) mean surface area

 **- 811.88x + 377200**

 **- 768.36x + 355395**

 **= 0.9205**

 **- 14.687x + 6819.6**

 **= 0.9697**

The mean diameters of grains and their mean areas change continuously and reveal lognormal layouts on the significance level of = 0.01 (λemp./ λ0.01 < 1). The exceptions were the treatment variants as follows: austenitization temperature of 910 oC and time - 3h for the mean area (fulfilled for lower significance level of = 0.001) and the temperature of 970 oC and time - 3h for both: mean diameter and mean area of prior austenite grain. Within the range of austenitization temperatures: 910 ÷ 940 oC for holding times: 3 and 5 hrs, the mean diameters and areas of prior austenite grain do not reveal any considerable differences. (Fig. 9). The values of the mean diameters and areas for this range of austenitization amounted to: 9.05 11.34 m and 94.03 144.72 m2, respectively. The above-mentioned

900 910 920 930 940 950 960 970 980 **Temperature of austenitization,oC**

**y = 0.0072x<sup>2</sup>**

900 910 920 930 940 950 960 970 980 **Temperature of austenitization, oC**

**R2 = 0.9946**

 **- 13.388x + 6221.5**

heterogeneity in terms of grain sizes, which are the lowest for this range of austenitizing temperatures (Fig. 10), and by the distributions of frequency of cumulated grains (Fig. 11).

 **b)**

coefficient, specifying

depending on the time of holding (3hrs – orange, 5hrs – black) of L21HMF cast steel

**y = 0.4369x<sup>2</sup>**

**y = 0.0079x<sup>2</sup>**

**R2 = 0.8959**

**y = 0.4154x<sup>2</sup>**

0

200

400

**Mean area,** 

 **m 2**

600

800

1000

0

**a)**

5

**M ean diam**

10

15

 **eter, m**

20

25

**R2**

**R2**

measurements can be confirmed by the calculated values of

**Table 4.** The results of measurements and calculations of the size of prior austenite grains for the cast steel

**Figure 8.** Logarithm – normal layout of grains for : a) mean diameter; b) mean surface area

28 Heat Treatment – Conventional and Novel Applications

**Min. area of grain, m2**

**Max. area of grain, m2**

**Figure 8.** Logarithm – normal layout of grains for : a) mean diameter; b) mean surface area

**Mean area of grain, m2**

910/3 976 4 695 142.67 204.58 1.74 1.63 1.067 925/3 969 3 741 104.27 173.60 1.27 1.63 0.779 940/3 954 3 810 121.34 172.77 1.40 1.63 0.859 955/3 964 3 1302 187.41 379.31 1.59 1.63 0.975 970/3 2024 3 95722 812.62 2138.56 1.99 1.63 1.221 910/5 946 3 608 101.07 162.93 1.07 1.63 0.656 925/5 915 2 741 144.72 248.95 1.61 1.63 0.988 940/5 959 3 842 94.03 147.10 1.00 1.63 0.613 955/5 937 2 1533 391.66 423.09 0.98 1.63 0.601 970/5 2034 3 102305 915.33 2408.43 1.53 1.63 0.939 **Table 4.** The results of measurements and calculations of the size of prior austenite grains for the cast steel

**Standard** 

**deviation emp =0.01**  .

**a)**

**b)**

**emp**

**α**

**λ λ 0 01**

**Amount n** 

**Heat treatment parameters, oC/h** 

**Figure 9.** Influence of austenitization temperature on: a) mean grain diameter; b) mean surface area depending on the time of holding (3hrs – orange, 5hrs – black) of L21HMF cast steel

The mean diameters of grains and their mean areas change continuously and reveal lognormal layouts on the significance level of = 0.01 (λemp./ λ0.01 < 1). The exceptions were the treatment variants as follows: austenitization temperature of 910 oC and time - 3h for the mean area (fulfilled for lower significance level of = 0.001) and the temperature of 970 oC and time - 3h for both: mean diameter and mean area of prior austenite grain. Within the range of austenitization temperatures: 910 ÷ 940 oC for holding times: 3 and 5 hrs, the mean diameters and areas of prior austenite grain do not reveal any considerable differences. (Fig. 9). The values of the mean diameters and areas for this range of austenitization amounted to: 9.05 11.34 m and 94.03 144.72 m2, respectively. The above-mentioned measurements can be confirmed by the calculated values of coefficient, specifying heterogeneity in terms of grain sizes, which are the lowest for this range of austenitizing temperatures (Fig. 10), and by the distributions of frequency of cumulated grains (Fig. 11).

Regenerative Heat Treatment of Low Alloy Cast Steel 31

At the temperature of 970 oC the grain growth was observed – mean diameter increased over two times, while the mean area about five times in comparison with the temperature

In order to determine the influence of cooling rates, allowing proper selection of parameters for the regenerative heat treatment, the TTT curves were plotted for L21HMF cast steel. On the basis of results achieved by means of dilatometric tests, a graph was drawn up, as shown in Fig 12. It illustrates the influence of cooling rate in the temperature range of 800 ÷ 500 oC on the structure and hardness of the investigated cast

**2.3. Determining the influence of cooling rate on the microstructure and** 

**Figure 12.** Influence of the cooling rate on structure and hardness of the cast steel

Analysis of the curves presented in Fig. 10 allows to state that in the case of L21HMF cast steel, whose chemical composition is given in Table 1, austenite cooled at 0.004 K/s ≤ v8-5 ≤ 0.017 K/s gets transformed into ferrite and pearlite. The rate of cooling for austenite: 0.023 < v8-5 ≤ 0.869 K/s makes it possible to obtain ferritic – pearlitic – bainitic structures. Whereas after cooling of the cast steel at the range of 0.869 K/s < v8-5 ≤ 14.630 K/s bainitic – ferritic structures were obtained, with an increasing bainite volume fraction as the cooling rate increased. Bainitic structure with around 6% volume fraction was received for the cooling

range of 910 940 oC.

rate of v8-5 14.630 K/s.

**properties** 

steel.

**Figure 10.** Interrelation between the heterogeneity factor (ν) of former austenite grain size in the cast steel and the austenitizing parameters for: a) mean diameter; b) mean grain area

**Figure 11.** Distributions of frequency of cumulated grains

At the temperature of 970 oC the grain growth was observed – mean diameter increased over two times, while the mean area about five times in comparison with the temperature range of 910 940 oC.
