Printing Hard Steels

[Corinthian]

What do you make of this?

 

http://https://phys.org/news/2020-04-uncover-art-extremely-hard-steels.html?fbclid=IwAR2KWGs5iWxsFrcb-LsniJW0AQt4XdZb-eVz-HGCXM7e_uq64w5wiDDbhrE

 

For millennia, metallurgists have been meticulously tweaking the ingredients of steel to enhance its properties. As a result, several variants of steel exist today; but one type, called martensitic steel, stands out from its steel cousins as stronger and more cost-effective to produce. Hence, martensitic steels naturally lend themselves to applications in the aerospace, automotive and defense industries, among others, where high-strength, lightweight parts need to be manufactured without boosting the cost.[/url]

[Loopycrank]

OK, so the article is a bit of a mess, but I think I get what they're getting at.

Part of the reason that steel is used for everything is that carbon and iron are abundant on Earth's surface, and another reason is that an iron/carbon alloy can have several different microstructures with substantially different macroscopic properties. Often, it is possible to switch the microstructure of a part just with heat treatment.

 

Martensite is nothing new. It's really old, actually. Think of a historical film about the Middle Ages. You know when the blacksmith thrusts the red-hot sword he's forging into water? That quenching process is to convert the steel of the sword from a ferritic microstructure into martensitic one. As the article notes, martensite only forms if the temperature transition occurs rapidly. Water is conductive enough that it sucks the heat out of the blade quickly, and the sword blade becomes the hard, wear-resistant martensite. Usually a purely martensitic structure is a bit too brittle for most uses, so the sword is usually tempered after quenching to create a tempered martensite microstructure, which is a little softer than pure martensite, but a whole lot tougher and less likely to shatter. There are some odd exceptions; traditional katanas have a pure martensite edge AIUI.

Tank armor is usually also tempered martensite. But there is a problem. Usually a tank hull needs to be welded together. Welding is hot enough that it can disrupt the heat treatment of the armored plates. So, you may start with armor plates of the finest tempered martensite, but when you weld them, the entire rest of the tank hull effectively acts as a giant heat sink and sucks the heat out of the weld quickly enough that the weld material turns into the brittle pure martensite. So it may be necessary to pre-heat the area around the weld, tweak the weld filler rod metallurgy or re-heat-treat the weld and the area around it after welding. This is why armor is a bit weaker at the weld and in the area immediately surrounding the weld (the heat affected zone, HAZ). If you look at pictures of knocked out WWII German armor, you'll see that the armor plate frequently breaks at the welds.

So, what I think is going on is that they're selective laser sintering little blobs of steel. The little blobs are welded together with the laser to build up the part. Each individual blob cools quickly enough that it forms martensitic microstructure. The fancy computer modelling allows them to optimize laser settings so that the printed part doesn't have a bunch of little micro-voids in it.