ingot micrographs

Micrographs of the interrupted melt ingots from a few years ago. The raw material was either mild steel or A36.

For the most part the mild steel is melting and falling unchanged into the hearth where it is carburized via contact with charcoal or furnace gases. In the following pictures the areas labeled 'low carbon cast steel' have the carbon content of the mild steel rods that were melted. Iron that has been in the hearth for a longer period of time and/or farther from the air blast has picked up enough carbon to completely liquefy and when cooled shows the typical structure of cast iron. Between the low carbon and cast iron areas there is a carbon diffusion zone with the carbon content grading between the two extremes.

Experiment DP1 (the shortest melt)

Macrophotograph of the ingot section

The upper region of the sample (lighter) has the typical structure of low carbon cast steel (see micrographs 1,2,3) The lower region of the sample (darker) is largely steel with approximately .7% Carbon content (micrographs 4 and 5). There is a band of white cast iron (micrographs 6 and 7). The diffusion of carbon from the cast iron to the low carbon area is illustrated (micrograph 8).

Map of micrograph locations

Micrograph DP1-1

typical low carbon cast steel structure (~.2% C)

austenite grains are approximately 5mm in diameter with grain boundary ferrite (thick white line) and widmanstatten ferrite (white angular) remainder pearlite(dark).

image width 1mm

Micrograph DP1-2

A small edge of the ingot shows a honeycomb structure where the iron was burning. In the flat white regions the carbon has been burned out of the iron leaving a trail of bubbles.

image width 2mm

Micrograph DP1-3

This a decarburized region adjacent to the previous micrograph

width 2mm

Micrograph DP1-4

pearlite with grain boundary ferrite around the austenite grains and a little widmanstatten ferrite.

~.6% carbon

width 2mm

Micrograph DP1-5

pearlite region with grain boundary ferrite around the austenite grains and widmanstatten ferrite.

carbon is a bit lower on the right side

width 2mm

Micrograph DP1-6

This photo is a little more interesting. The white region was liquid cast iron of close to 4% CE (Carbon Equivalent) showing the structure of ledeburite and a few austenite dendrites solidifying from the melt (small circular pearlite regions in the top right of the image). In the center we have carbon diffusing from the cast iron into austenite which on cooling becomes pearlite colonies with grain boundry and needle cementite.

width 1.5mm

Micrograph DP1-7

a closer view of the liquid solid interface

width 0.5mm

Micrograph DP1-8

The carbon diffusion zone. The region at the lower right is cast iron. The region at the upper left is the edge of a low carbon cast steel area.

width 3mm

Experiment DP2

ingot sliced

The upper region of the sample (lighter) has the typical structure of low carbon cast steel (see micrographs 1,2,3) The lower region of the sample (darker) is largely pearlite (micrographs 4 and 5) with large patches of white, mottled, and pearlitic-grey cast iron. The voids contained charcoal fragments.

Map of micrograph locations

Micrograph DP2-1

Low Carbon Cast Steel

grain boundary ferrite, widmanstatten ferrite, pearlite (dark). ~0.2% C

width 1mm

Micrograph DP2-2

Low Carbon Cast Steel

pearlite island in low carbon cast steel region.

width 1mm

Micrograph DP2-3

Low Carbon Cast Steel

width 1mm

Micrograph DP2-4

The grain boundary between higher and lower carbon (upper right) austenite grains.

width 2mm

Micrograph DP2-5

Carbon diffusion zone

pearlite with ferrite (upper left), pearlite with cementite needles (lower right)

width 2mm

Micrograph DP2-6

Mottled cast iron

The eutectic ledeburite (white) surrounds austenite dendrites. The black squigglies are graphite. This structure is the the result of having both manganese and silicon in the iron and the rate of cooling from the liquid state.

width 1.5mm

Micrograph DP2-7

typical white cast iron

width 0.5mm

Micrograph DP2-8

Interface between grey cast iron and pearlite at the edge of a void in the ingot.

width 3mm

Panorama

Composite of four images covering the interface between a high carbon liquid region (left) to low carbon ferrite region (right).

width 4mm