ferrites pearlites austenites

These are the three words that any metallurgist or materials scientist sees constantly. But when you start working with real metal, especially in the production of equipment, you realize that there is an abyss between the ideal state diagram and what lies on the table in the workshop. Many people, especially young engineers, think that they have understood the theory - and that’s it, they can predict properties. But in fact, the sameperlitein the casting of the crusher body and in the rolled beam - these are, excuse me, two big differences. Or let's takeaustenite— everyone knows about its stability in stainless steels, but when you make large-sized components for mining equipment, where impact strength at low temperatures is important, you begin to value austenitic steels in a completely different way, you look for a balance with ferrite so that cracks do not appear from thermal shock.

From theory to workshop: where problems begin

I remember about ten years ago we tried to modernize a gearbox at one of the old factories. The customer required increased wear resistance of the gears. We looked at the material - steel with a good carbon content; in theory, a good fine-plate structure should have formedperliteafter appropriate heat treatment. We carried out normalization, hardening, and tempering. On a microsection under a microscope there seems to be a beautiful structure. But during the first tests on the stand, the tooth began to crumble. They began to figure it out.

It turned out that the history of metal did not begin in our furnace. The original billet was overheated during rolling, and traces of Widmanstätt remained in the structure. Our heat treatment did not correct this, it simply “camouflaged”. The pearlite has formed, but there are still brittle areas around it. So much for pure theory. It was necessary to completely change the technological chain, introduce additional homogenization exposure in order to level the structure before hardening. Expensive and time-consuming, but the result was successful.

This incident taught me to take a broader view. Now thatLONGI CorporationWhen the task comes to develop or repair a critical component, for example, the shaft of a magnetic separator, which operates under conditions of abrasive wear and variable loads, we ask not only the steel grade. We need a story: what deformation it underwent, how it cooled. It depends on this whether we can obtain the desired ratio of ferrite and pearlite in the final product or whether we will have to go for more complex solutions.

Austenite: not only for stainless steel

Austenite is also full of myths. Everyone immediately remembers corrosion resistance. But in mining equipment that producesLONGI, strength and vibration damping are often critical. For example, we have attachment points for power electromagnets. The vibration from a working crusher or mill is colossal. Using ordinary carbon steel here means dooming the structure to fatigue failure.

We tried to use steels with metastable austenite, which partially transform into martensite during deformation. The effect of hardening by work hardening. In theory - great. In practice, we encountered the fact that the degree of conversion strongly depends on the temperature of the unit itself during operation. In the summer in a hot workshop there is one result, in the winter in an unheated room - another. Unpredictability is the worst thing about engineering.

As a result, for production cars we often abandon such delights in favor of more predictable, albeit less “fashionable” ones. materials. But for special orders, where conditions can be strictly controlled, we continue experiments. The balance between austenitic ductility and ferrite-pearlite strength is a constant search.

Ferrite as a base

Ohferriteoften spoken disparagingly: soft, weak. But you can’t go anywhere without him. This is the phase that provides that very viscosity, the ability to absorb impact energy. In buckets and excavator teeth that experience shock loads, the presence of a certain volume of soft ferrite in the structure is not a disadvantage, but a necessity. It slows down the crack.

The problem is how to control it. The cooling rate after forging or casting is a key parameter. Previously, in the old production, a lot was done “by eye”: they took it out of the oven, waited until the color stopped being bright red, then put it in water or air. The result varied from game to game. Now on a modern siteLONGI Corporationin Fushun they are trying to avoid this. Controlled cooling lines are being introduced, especially for large castings. Because a cooling 10-ton frame and a small bushing are different universes from the point of view of the kinetics of phase transformations.

There was an unpleasant incident with a batch of screening slabs. The material seemed to be standard, but during installation several slabs burst from a slight impact. The microstructure showed huge, elongated ferrite grains forming an almost continuous network along the boundaries of the former austenite grains. Perlite inside the grains. Fragile structure. The reason is too slow cooling in a certain temperature range. The furnace malfunctioned, and the control was selective. The entire batch had to be melted down. Losses, of course. But such experience is worth a lot - it forces you to build a control system not only at the end points, but also along the entire heat treatment path.

Perlite: subtleties of dispersion

Perlite is probably the most problematic when it comes to hardness and wear resistance. Everyone wants to get thin-plate, even sorbitol-like perlite. The smaller, the harder. But this is where technology comes into play. To obtain very fine perlite, a strictly defined and sufficiently high cooling rate is needed. For a massive part, this can mean sudden cooling in oil or even in a polymer solution, which is fraught with high residual stresses and warping.

This is critical for the crusher shafts we make. Curvature of even a couple of tenths of a millimeter per meter of length can lead to imbalance and destruction of bearing units. Therefore, we often make a compromise: we allow slightly larger perlite, but carry out isothermal treatment, for example, in a salt bath at a temperature slightly above the onset of martensite formation. We get bainite. The structure is more complex, but its complex properties (strength, toughness, minimal warping) are often superior to pure pearlite.

This is not from a textbook, where everything is clearly laid out on the shelves. This is the kitchen, where the recipe is adjusted to fit the existing stove and pots. ExperienceLONGI Corporation, which has been accumulating since 1993, is precisely the accumulation of such “recipes?” for different types of mining equipment. A total area of ​​140,000 m2 and more than 1,200 employees are not just numbers, it is an opportunity to conduct such experiments and accumulate statistics, which then form the basis of technological maps.

Synthesis in a specific product

As a result, when you design a new machine, for example, a magnetic separator, you do not think separately about ferrite, pearlite or austenite. You think about functional areas. A drum that is constantly in contact with the pulp requires maximum resistance to abrasive, which means an emphasis on solid phases, possibly surface hardening to martensite. The frame, which bears the main load, is where strength and rigidity are important, which means high-quality alloy steel with a guaranteed fine, uniform structure after volumetric hardening and high tempering. And some mounting brackets, where plasticity is important to dampen vibrations, can be left with a more ferritic structure.

It is this systematic approach that allows the company to produce about 4,000 units of equipment per year with predictable characteristics. Each phase in a state diagram ceases to be an abstraction and becomes a tool. A tool that you need to be able to use, knowing all its vagaries. Sometimes the best result is not the phase that everyone praises in textbooks, but the one whose behavior you have learned from your own mistakes and successes. This, perhaps, is the main thing that distinguishes theory from practice in our work with metal.