With a bit of help from my workmates, I’ve got my hands on some interesting metal samples which I’ve etched and taken some photos of through my microscope.
Below are some photos of a Martensitic high chromium white cast iron that show evidence of heat checking (cracking due to thermal fatigue, IE, hot grinding)
I’m not 100% sure on what the microstructure is but from what I’ve read (ASM Specialty Handbook: Cast Irons), but what I think I’m seeing below is dendrites of Austenite-Martensite in a matrix of Cementite. The Austenite apparently doesnt 100% convert to Martensite on quenching, though sub-zero heat treatment does get most of it to convert. These samples have had such treatment.
You can see the dendrite formation in these two photos.
Below are photos of pieces of Austenetic Manganese Steel that have been fillet welded together using some various techniques. The first is with mild steel filler wire, air cooled, no preheat. There is some porosity present as the welder’s gas solenoid started to play up mid weld.These samples were all initially etched with hydrochloric acid (from a pool shop). Since the hydrochloric didn’t etch the stainless samples shown later, I also tried sulphuric (car battery) oxalic acid, phosphoric acid (rust converter), hydrochloric acid + methanol, hydrochloric acid + some other solvent (brick cleaner), boric acid (borax) and garlic salt (I didn’t have any normal salt). I’d really like to try some Nital or Picral but I can’t seem to find anywhere in Melbourne that even knows what it is.
The upper and lower sections with visible grains are the manganese steel. The porours right hand side is the mild steel filler. The shiny bit on the left is a stainless steel tack weld.
The big stain in the middle is a crack. The phosphoric acid really brought that out.
Next is the same thing, but quenched in water after welding. Normally, quenching a weld is a big no-no but manganese steel is different. It’s got lots of manganese in it (11-14% or so) which stops carbides from precipitating from the Austenite. The result of this is that it can be quenched, resulting in an Austenitic structure at room temp. It’s soft but work hardens a lot and very quickly. According to the (super awesome) webpage: George’s Basement, the mechanism behind this is something along the lines of this material forms very long stacking faults that interfere with each other and kind of bind up. I think.
Anyway, quenching Mang is OK but the mild steel filler has probably gone Martensitic. Unfortunately, this microscope doesn’t have the magnification to see Martensite’s structure. I might need to get my hands on my old scope which is currently in service looking at dead fish. True story.
There appears to be a third kind of metal in here. Damned if I know how. Did the stainless and the mild steel alloy together?
You can see here that the quenched sample cracked also.
Next is stainless filler, air cooled. You can see on this one that there is some kinda cross-alloying taking place at the border between the weld and the mang.
Finally we have the same thing, quenched. I can’t see any discernible difference between them except I did a particularly bad job of polishing this sample.
You can really see the grains of the manganese steel in this photo.
OK, so that’s all the shots I have that are worth showing. Next, I’m going to try to etch them with Hydrofluoric acid and then try to figure out a way of putting a measurable load on the samples to see how strong they all are. I’ll post more pics then.