November 2020 Volume 2

FORGING RESEARCH AND TECHNOLOGY

FORGING RESEARCH

WQ samples, a martensitic phase distribution could be observed. Figure 27 shows the SEMmicrographs for the Wrought 4340 steel. microstructure with Cr-carbides along the grain boundaries and a martensite-austenite (MA) phase. As the cooling rate was increased, the microstructural components for the wrought sample cooled at 20°C/min and at 100°C/min were simple: martensite and bainite. For the WQ samples, a martensitic phase distribution could be observed. Figure 27 shows the SEM micrographs for the Wrought 434 steel. In the other hand, for the wrought 4340 sample, cooled at 1°C/min after deformation also exhibited a ferrite-pearlite

3.5Microstructural Analysis after Hot Compression The transformation behavior of deformed austenite under continuous cooling conditions provided a wealth of microstructural information. The results from the microstructural characterization of the 4340 SLM and 4340 wrought samples will be described in the following sections. The results showed differences in terms of microstructural constituents, phase character distribution and micro-hardness values. For the SLM 4340 sample, cooled at 1°C/min after deformation exhibited a ferrite-pearlitemicrostructure withCr-carbides along the grain boundaries and also a martensite-austenite (MA) phase. With the increase of the cooling rate, the microstructural components for the SLM sample cooled at 20°C/min were martensite, bainite, granular bainite (GB) and martensite-austenite (MA) and, for the SLMsample cooled at 100°C/min, the components were martensite, bainite and carbides-free bainitic ferrite. A martensitic phase distribution was observed for both water-quenched samples, prior to and after deformation. Figure 26 shows the SEM micrographs for the SLM 4340 steel. FORGING RESEARCH ch racter distribut on and micro-hardness values. For the SLM 4340 sample, cooled at 1°C/min after defo mation exhibited a ferrite-pearlite microstructure with Cr carbides along the grain boundaries and also martensite-austenite (MA) phase. With the increase of the cooling rate, the microstructural components for the SLM sample cooled at 20°C/min were martensite, bainite, granular bainite (GB) and martensite-austenite (MA) and, for the SLM sample cooled at 100°C/min, the components were ma ensite, bainite and carbides-free bainitic ferrite. A martensitic phase distribution was observed for both water-quenched samples, prior to and after deformation. Figure 26 shows the SEM micrographs for the SLM 4340 steel.

MA

Martensite

Pearlite

Ferrite

Bainite

2 µm

2 µm

( A )

( B )

Martensite

Martensite

Bainite

2 µm

2 µm

( D )

( C )

Martensite

MA

Martensite

Bainite

Pearlite

2 µm

Granular Bainite

Ferrite

Figure 27: SEMmicrographs for the Wrought 4340 cooled at (A) 1°C/min, (B) 20°C/min, (C) 100°C/min. (D) WQ prior to deformation and (E) WQ after deformation. In summary, a comparison of the microstructures for the two steels study seems to provide a clear impact of the cooling rate effect. For the SLM 4340, conventional microstructural components were observed at the slowest cooling rate, i.e. ferrite, pearlite, MA and Cr-carbides. When the cooling rate was increased after deformation, more complexes phases could be seen, especially for the cooling rate of 20°C/min. In this condition, martensite, bainite, granular bainite and MA could be observed throughout the sample. At the fastest cooling path, most of the SLM sample exhibited martensite and bainite, except for the presence of carbides-free bainitic ferrite. As expected, the water quenched samples were 100%martensitic. For the wrought 4340, martensite and bainite were observed at cooling rates higher than 20°C/min. At this level of microstructural analysis, the wrought sample cooled at 1°C/min had the same behavior as the selective laser melted sample, according to phase distributions, i.e. ferrite, pearlite and MA. Again, as expected, for the water quenched samples, only martensite could be seen. Figure 28 shows a comparison for both alloys cooled at 20°C/min. The main difference between the samples is the presence of granular bainite in the SLM sample. 23 24 ( E ) Figure 27: SEM micrographs for the Wrought 4340 cooled at ( A ) 1°C/min, ( B ) 20°C/min, ( C ) 100°C/min. ( D ) WQ prior to deformation and ( E ) WQ after def rmation. In summary, a comparison of the microstructures for the two steels study seems to provide a clear impact of the cooling rate effect. For the SLM 4340, conventional microstructural components were observed at the slowest cooling rate,

2 µm

2 µm

( A )

( B )

Bainite

Martensite

Martensite

Carbides-Free Bainitic Ferrite

2 µm

2 µm

( C )

( D )

Martensite

2 µm

Figure 26: SEM micrographs for the SLM 4340 cooled at (A) 1°C/min, (B) 20°C/min, (C) 100°C/min. (D) WQ prior to deformation and (E) WQ after deformation. In the other hand, for the wrought 4340 sample, cooled at 1°C/min after deformation also exhibited a ferrite-pearlite microstructure with Cr-carbides along the grain boundaries and a martensite austenite (MA) phase. As the cooling rate was increased, the microstructural components for the wrought sample cooled at 20°C/ min and at 100°C/min were simple: martensite and bainite. For the ( E ) Figure 26: SEM micrographs for the SLM 4340 cooled at ( A ) 1°C/min, ( B ) 20°C/min, ( C ) 100°C/min. ( D ) WQ prior to deforma ion and ( E ) WQ after def rmation.

FIA MAGAZINE | NOVEMBER 2020 94