November 2020 Volume 2

FORGING RESEARCH AND TECHNOLOGY

FORGING RESEARCH

FORGING RESEARCH

Figure 16 shows the average bulk micro hardness (HVN) values attained. These measurements were performed to assess the impact of different cooling rates on the resulting microstructures. The HVN results indicated that the microstructures obtained from the Wrought 4340 steel were harder than those from the SLM steel for a given cooling rate. The hardness results can be rationalized in terms of the microstructural conditions of the Austenite grain size and its chemical composition prior to its transformation. The effect of the chemical composition will be addressed in a separate publication. cooling rate. The hardness results can be rationalized in terms of the microstructural conditions f the Austenite grain size and its chemical composition prior to its transformation. The effect of the chemical composition will be addressed in a separate publication.

Granular Bainite

Granular Bainite

Figure 16 shows the average bulk micro hardness (HVN) values att ined. These measurements were perform d to assess th impact of different cooling rates on the resulting microstructures. The HVN results indicated that the microstructures obtained from the Wrought 4340 steel were harder than those from the SLM teel for a given

1 µm

1 µm

MA

MA

( C )

( D )

Martensite

Carbides Free Bainitic Ferrite

0 100 200 300 400 500 600 700 800 Hardness (HVN)

Bainite

Bainite

Granular Bainite

1 µm

1 µm

( F )

( E )

Carbides Free Bainitic Ferrite

Martensite

Figure 16: Average bulk hardness values for SLM 4340 and Wrought 4340 steel. 3.3Microstructural Analysis after Dilatometry 3.3.1 SLM4340 Steel The transformation behavior of Austenite under uniformcontinuous cooling paths provided a wealth of microstructural information. For example, at the slowest cooling rate, i.e. 0.25C/min, the microstructure consisted of Ferrite-Pearlite with Cr-carbides along the grain boundaries. As the cooling rate was increased, other low temperature transformation products were observed. These were Martensite-Austenite (MA) and Granular Bainite, along the prior Austenite grain boundaries. At cooling rates between 50˚C/min and 100˚C/min, Bainite, Martensite and Carbides Free Bainitic Ferrite were observed. Figure 17 shows examples of the typical SEM microstructures identified. Figure 16: Average bulk hardness values for SLM 4340 and Wrought 4340 steel. Microstructural Analysis after As the cooling rate was increased, other low temperature transformation products were observed. These were Martensite-Austenite (MA) and Granular Bainite, along the prior Austenite grain boundaries. At cooling rates between 50 ˚ C/min and 100˚C/min, Bainite, Martensit and Carbides Free Bainitic Ferrite were observed. Figure 17 shows examples of the typical SEM microstructures identified. SLM 4340 Wrought 4340 F rr MA Cr-carbides Cr-carbides

3.3.

1 µm

1 µm

Bainite

Dilatometry 3.3.1.

SLM 4340 Steel The transformation beh vi r of Austenite under unif rm con inuous cooling paths provided a wealth of microstructural information. For example, at the slowest cooling rate, i.e. 0.25C/min, the microstructure consisted of F rrite-Pearlite with Cr-carbid s along the grain boundaries.

Figure 17: SEM pictures of (A) 0.25C/min, (B) 1C/min, (c) 5C/min, (D) 10C/ min, (E) 20C/min, (F) 50C/min, (G) 100C/min, (H) 500C/min; all samples were etched with 3%Nital. 3.3.2Wrought 4340 Steel The SEM microstructural analysis of the Wrought 4340 samples after uniform continuous cooling showed a substantially more complex microstructural distribution compared to the microstructures observed in the SLM samples. At slower cooling rates, i.e. 0.25C/min, 0.5C/min and 1C/min, the microstructures consisted of Ferrite-Pearlite, Martensite-Austenite (MA) and Granular Bainite (GB). At higher cooling rates (≥ 5C/min) the main microstructural components were Bainite andMartensite. Figure 18 shows SEM micrographs of the microconstituents in the Wrought steel samples. ( H ) Figure 17: SEM pictures of ( A ) 0.25C/min, ( B ) 1C/min, ( c ) 5C/min, ( D ) 10C/min, ( E ) 20C/min, ( F ) 50C/min, ( G ) 100C/min, ( H ) 500C/min; all samples were etched with 3% Nital. 3.3.2. Wrought 4340 Steel The SEM microstructural analysis of the Wrought 4340 samples after uniform continuous cooling showed a substantially more complex microstructural distribution compared to the microstructures observed in the SLM samples. At slower cooling rates, i.e. 0.25C/min, 0.5C/min and 1C/min, the microstructures consisted of Ferrite Pearlite, Martensite-Austenite (MA) and Granular Bainite (GB). At higher cooling rates (≥ 5C/min) the main microstructural components were Bainite and Martensite. Figure 18 shows SEM micrographs of the microconstituents in the Wrought steel samples. 15 ( G )

Pearlite

Ferrite

1 µm

1 µm

ides

( A )

( B )

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FIA MAGAZINE | NOVEMBER 2020 90