Stainless steel

• Model of Delong:

Contrary to the model of Schaeffler, the model of Delong takes into account nitrogen.

Types of stainless steels

The chrome steels are ferritic and magnetic in a softened state. Some behave like self-hardening special steels, others are soaked only partially or at all. The steels with nickel-chromium are in general austenitic and the treatment of hyper-hardening, far from hardening them, has on the contrary the property to soften them. There exist innumerable nuances appropriate to the most various uses.

With regard to the use, one distinguishes martensitic, ferritic and austenitic steels.

• the martensitic steels are used when the properties strength mechanical are important. Most current contain chromium 13% with at least carbon 0,08%. Other nuances are charged in additions, with possibly a small percentage of nickel.
• the ferritic steels do not take hardening. One finds in this category of the heat-resisting steels with high chromium content (up to 30%), particularly interesting in the presence of sulfur.
• the austenitic steels are most numerous by far, because of their very high chemical resistance, their ductility comparable with that of copper or brass, and also of their good high mechanical characteristics. The contents of elements of addition turn around 18% from chromium and nickel 10%. The percentage of carbon is very low and the stability improved by elements such as titanium or the Niobium.
• the austeno-ferritic steels have intermediate properties between the two preceding categories and among them alloys particularly suited to the welding and others are very corrosion resistant intergranular.

The knowledge of the types of steel is essential for the systems made up of assembled elements mechanically or by welding, the setting in the presence of two too different stainless steels in an electrolyte can indeed cause very destroying phenomena of electrochemical corrosion.

Conditions to join together to support the corrosion resistance

The factors favorable to the fight against corrosion are also applicable to the stainless steels:

• surfaces must be pickled to eliminate all oxides resulting from hot work: rolling, forging, heat treatments, welding joinings, etc,
• to treat only clean and dry parts thermically, without traces of greases, residues of grease-removing products, and especially without ferrous particles. Cleaning with the nitric acid before treatment is generally an excellent solution,
• Supprimer the residual stresses resulting from a cold work hardening, in particular those which result from stamping,
• Éviter, when designing part, to create zones difficult to clean,
• Éviter all the nonessential contacts between the stainless steel parts and the other materials, metal or not,
• Plus still for the stainless steels than for other metals, the surface quality must be particularly neat because it conditions the establishment of a passivating film.

Influence various mediums

• industrial Water: the pure water is without effect but the chlorides (and to a lesser extent much of other salts), even with the state of traces, are particularly harmful for the stainless steels; the nuances containing of molybdenum then are indicated.

• Steam: normally without effect, it can however pose problems if it contains certain impurities.
• Natural atmospheres of a furnace, except for the marine atmospheres: they pose of as much less problems than steel contains more noble elements and than surface is better polished.
• marine and industrial Atmospheres: the chrome steels deteriorate very slowly but one in general prefers to use steels molybdenum.
• Acid nitric: it tackles the majority of industrial metals but the stainless steel in general resists to him particularly well, in consequence of the passivation of its surface: molybdenum is not interesting that if the acid contains impurities.
• Acid sulphuric: resistance depends much on the concentration and the presence of oxidizing impurities improves passivation. Generally the austenitic nuances containing of molybdenum are the best.
• Acid phosphoric: resistance is generally good but the impurities should be supervised, in particular the hydrofluoric Acid .
• acid Sulphites: corrosion can be catastrophic because these solutions, which one often meets in the Papeterie S, comprise many impurities; there still the alloys with molybdenum are preferable.
• Hydrochloric acid: corrosion increases regularly as the concentration increases, association is thus to avoid.
• Organic acid: they are generally not as corrosive as the mineral acids and those which one meets in food industry (acid acetic, oxalic, citric, etc) are practically without effect.
• alkaline Solutions: the cold solutions practically do not have an action but it is not the same for the concentrated and hot solutions.
• Saline solutions: the behavior is generally rather good, except in the presence of certain salts like chlorides; the Nitrate S on the contrary support passivation and improve the behavior.
• Food products: there is generally no safe problem of corrosion with certain products which contains natural or added sulfurous components, like the mustard and the white wines.
• Material organic: they are generally without action on the stainless steels, safe if they are chlorinated: the Adhesive S, Soap S, Tar S, oil products, etc do not pose any problem.
molten
• Salts and other mineral products: the alkaline products corrode all the stainless steels but the nitrates, Cyanure S, Acétate S,… do not attack the stainless steels. The majority of other salts and the molten metals produce fast damage.

Placement of the stainless steels

Particular problems of hot work

• They react enormously to the rather high temperatures,
• They are very bad conductive of heat,
• Their mechanical resistance is high, especially in the case of the austenitic ones (cold they are on the contrary martensitic the most resistant),
• the grain tends to grow bigger hot and can be regenerated only by welding,
• work must be followed by an annealing and a scouring making it possible to benefit from the corrosion resistance.

The massive parts must thus be heated slowly until approximately 800°C before being carried more quickly at the temperature of work, which is at the neighborhoods of 1000°C. It is necessary to avoid before all the decarburization of martensitic steels, the prolonged maintenance at high temperature of ferritic steels and of the austenitic steels, whose grain grows bigger easily and appears difficult or sometimes even impossible to regenerate. Fast cooling with water, after work, is often recommended.

Heat treatments

It is generally in the form of sheets or of tubes that one uses the stainless steels, and in this case one is often obliged to practice a Recuit of softening after operations such as the Emboutissage, to avoid the maintenance of too high residual stresses.

Degreasing before treatment must be particularly neat, the oxidizing atmospheres are indicated and the containing hydrocarbon atmospheres must be proscribed.

Martensitic steels find their principal use in mechanical engineering, in the form of massive parts. To obtain wanted resistance, they are generally soaked then returned S. softening generally is essential after the work hardening resulting from cold work. The income lowering resistance to the Corrosion, it is better to use a nuance less rich in carbon which decreases the intensity of hardening and makes it possible to avoid an income at too high temperature.

Ferritic steels do not take hardening but they often should be reheated, for example between two master keys of stamping or in the event of welding. A too long maintenance with high temperature generates a certain brittleness in consequence of the enlargement of the grain.

Austenitic and austeno-ferritic steels are softened by a treatment high temperature, of 900°C until 1150°C, followed by a cooling as fast as possible. The corrosion resistance, particularly with its intergranular form, requires to practice as much as possible a treatment of hyper-hardening.

The relaxation of the internal tensions can be done at relatively low temperature, approximately 400 or 450°C.

The stainless steels with age hardening require particular treatments according to the nuances.

Cold forming

All the usual techniques of cold work are applicable to the stainless steels and thus to the parts obtained starting from sheets or from wire that one can find in innumerable objects of everyday usage.

The stainless steels are relatively hard and this hardness rises by work hardening, as they are deformed. This phenomenon is particularly marked for the austenitic steels. Ferritic steels are hammer-hardened less but the lengthening which one can impose to them is weaker.

The “elastic return” after forming is much larger than for the mild steels “ordinary”.

The Lubrication between the parts in the course of forming and the tools is essential but does not pose particular problems for the majority of the operations. However, for the parts in decorative matter it is necessary to pay attention to the formation of surface defects in consequence of an inopportune seizing. The use of tools out of tempered steel, cast iron méhanite or out of aluminum bronze, as well as protections by varnishes pelables or sheets of plastic often constitute a good solution.

Work hardening decreases the corrosion resistance and creates sometimes a residual magnetism. An annealing makes it possible to restore the structures.

The Folding with the press or the serrated roller does not present a particular difficulty.

The Emboutissage requires machines twice more powerful than those which are useful for the mild steel. The pressure exerted by the blank holders must be sufficient to avoid crumplings but not too much to avoid the tears. The pig iron and cast iron allied with nickel-chromium gives best the tools, the thin sheets can be formed in alloy copper-zinc matrices. The Congé S must have a too small ray neither, nor too large, to avoid at the same time an excessive work hardening and crumplings, one takes in general between 5 and 10 times the thickness of the blanks. Lubrication is carried out with all the traditional lubricants, neuses solutions Savon, soluble oils or not, with in the difficult cases of the solid chemically active matter or lubricant additions: Lead, Talc, Graphite, Molybdenum disulfide, sulphuretted or sulfochlorées oils, additive phosphorated,… annealings are done preferably in oxidizing atmosphere and as much as possible at once after stamping.

The Repoussage does not pose a particular problem, the precautions to be taken are the same ones as for stamping, the best tools are out of cement steel.

Assembly of the stainless steels

Welding and brazing

The traditional methods of welding of the mild steels remain valid as a whole; one seeks healthy weldings naturally, without porosities, equipped with a good mechanical resistance, but here it is necessary moreover that they preserve qualities of corrosion resistance which are those of basic materials.

It is necessary for all to limit the chromium losses as far as possible, because of the crucial role of this element which, let us recall it, is very oxydable. One thus protects the weld beads by various means, the slag of coating of the electrodes, a neutral flame of blowtorch, or an inert atmosphere of gas (Argon for example). As for the heat treatments it is carefully necessary to degrease the parts and to proscribe all that could produce a carburation of metal.

• martensitic steels, because of their high percentage of carbon and their carbon are equivalent high, lend themselves badly to welding.

• ferritic steels tend to becoming fragile and must imperatively be annealings after welding if one wants to profit from good mechanical characteristics.
• the austenitic steels are most suited to welding but it is necessary to choose carefully the filler of the rods or the electrodes.

One always may find it beneficial to privilege the methods which limit in time and space the fusion of metal: the Welding resistance (by points, with the serrated roller, by flashing) gives excellent results and one should not forget the Brasage, which does not cause any fusion of steel itself. “Welding” with the tin gives very good performances provided that the parts are very carefully cleaned and that one carefully eliminates any trace from pickling solution at the end of the operation. The silver brazing gives very resistant joints but brazings with copper and tin are disadvised because they introduce a certain brittleness of the base metal. The best means to weld stainless remains the welding under inert atmosphere (argon) with station to weld of type MIG (Metal Inert Gas). Flow d´argon around the bath of welding protects this one to l´air surrounding, thus preventing l´apport gazs such as l oxygene or l hydrogene for example which can cause a cold cracking of the welding and prevents also the contribution of carbon contained in the air what would modify considerably the properties of stainless in fusion. (loss of its ductility, corrosion)

Riveting and bolting

The rivets give quite tight joints because of their high dilation coefficient. Below 5 mm one can rivet cold. The sealing is generally less good than for ordinary steels, because of the absence of rust.

It is of course advised not “to marry” metals in a disparate way, in order to avoid electrochemical corrosion that would not fail to cause. The stainless steel screws and bolts and nuts and bolts thus are essential quite naturally.

Machining

From the point of view of machining the stainless steels can be classified in two categories:

• ferritic and especially martensitic steels machine practically same manner as the traditional structural steels of the same hardness, it is however advised to reduce the cutting speeds slightly.
• the austenitic steels are distinguished from the ordinary structural steels by their weak elastic limit, their important lengthening before rupture and their strong aptitude for work hardening, which obliges to modify the conditions of machining in proportions sometimes very important. Generally it is necessary to use machines more powerful, very rigid, not vibrating, and to very vigorously fix the parts which one wants to work. One will privilege the strong depths of cut at relatively low speeds.

The angles of cut must be largest possible to accentuate the solidity of the edges and to facilitate the dissipation of the heat.

The liquids of cut play a particularly important part in the case of the austenitic steels. A very strong consistency (capacity of a lubricant to be fixed firmly at the walls in consequence of various phenomena of Adsorption) is necessary: one will thus use sulfur or sulfochlorées mineral oils added possibly with greasy substance as the castor oil or colza.

Cutting

Ferritic and martensitic steels are worked like steels current, but not the austenitic ones. Those have a strong propensity with seizing and it is necessary to take care of the good side skin saws and punches; the power of the machines must be definitely higher. In all the cases one will take care of well eliminating the damaged parts, particularly in the case of flame cutting.

Surface treatments

The surface treatment is used to improve the surface quality of a given material.

Scouring and passivation

It is necessary for all to eliminate all calamine, the more or less adherent ferrous particles following the passage in the tools of manufacture or brushing to the metal brush, the abrasive residues of tools (especially if they were before used to work of ordinary steels). Chemical scouring and sanding are highly advised.

It always should be taken care that the parts that one brings into service are suitably passivated, which can be made if one gives up them sufficiently a long time with the air or if are oxidized they chemically to save time.

Grinding and polishing

To avoid the contamination of surfaces, the tools of Grinding and Polissage must as much as possible being reserved with the work of the stainless steels. The lubricating films which are often formed during these operations must be carefully eliminated because they insulate metal and prevent its passivation.

Polishing is indicated only whenever it can really improve the surface quality, one can often occur some for cold rolled sheets.

As far as possible the quality of the weldings will be looked after so that one does not need to complete them by grinding, because this operation decreases their resistance.

Electrolytic polishing causes generally less losses of matter than mechanical polishing. However it must be led according to very strict regulations to give good performances.

Maintenance