Stainless Steel

 

It is part of the lives of thousands of people. It is present in the kitchens, sinks, cutlery and tableware. It is also present in electrical appliances, automobiles, busses and train wagons. In addition to facades, elevators, escalators, street furniture, and industrial kitchens, hospital appliances, capital goods equipment and industry in general. Beautiful, practical, versatile: stainless steel provides an outstanding performance and reaches a new market sector every day.

The applications of stainless steel are many, which can be confirmed by broad use, low maintenance and easy cleaning. So are the possibilities for new applications, which challenge the daring of the professionals not only for the visual but also for the economical and long-lasting aspects.

Definition

 

Metallic alloys based in Fe that contains at least 11% of Chromium. Increasing  the rate of Chromium and adding some elements, the stainless steel is produced  to accomplish a large range of resistance to corrosion.

 

FEATURES	Carbon Steel High or Low Alloy	Stainless Steel
Alloy type	Fe, C+¯ % [Cr, Ni, Mo, Mn]	Fe, C, Cr+ % [Ni, Mo, Mn]
Conformability Hot/cold	OK	OK
Melting	OK	OK
Change of mechanical properties	By metalworking or raw-making – OK	Raw-making ok metalworking but not – OK for martensitic
Resistance to corrosion	NO	OK

 

Types of Corrosion in Stainless Steel

 

Uniform or Generalized Corrosion: Is suppressed by the formation of a passive layer. Other types of corrosion might occur under certain conditions: Pites: is a localized corrosion, usually forming circle holes, that in a very short time go deeper in the material.

The Mo addition increases the resistance to the attack performed by chlorates, acting together to Cr.; extending the range of passivity. It can be evaluated by the equation: PRE=%Cr+3,3%Mo+(x)%N.

Where PRE = Pitting Resistance Equivalent X= 16 for duplex steel and 30 for austenitic steel.

Inter granular: is an attack along the corners of the grain that might take to the loss of inter metallic cohesion. The fall of Cr rich carbonates, of the corners and loss of Cr in the neighbor regions cause it.

How to avoid it? Reduction of %C under 0,03% Proper Thermal Treatments For ferritic: re cooking between 800 and 850ºC For austenitic: solve at 1050 Addition of stabilizers: Nb and Ti.

Corrosion under tension: under traction tensions (internal or external) and aggressive environment attacks (chlorates), failures might occur in austenitic steels. In ferritic steel and duplex steel it practically will not occur.

Galvanic corrosion: occur when two metals are in conductive contact through a liquid that conducts electricity. The material that is less valuable in the electro- chemical solves itself.

Chemical Compound Influence in Corrosion Resistance

 

Cr: Is the essential element in the formation of the passivity layer. Other materials can increase the effectiveness of Cr in the formation and maintaining of the layer, but none can substitute it. It is Ferrite stabilizer (restrains the austenitic field). At low temperatures (under 820ºC), the alloys with percentage Cr between 25 and 65%, it might occur the formation known as Sigma phase. It's presence in Cr rich ferrite is followed by hardening and extreme weakening of the alloys, and can be precipitated when submitted through a long period in a temperature range between 600 and 850ºC.

Ni: Effective about the re-passive (regeneration of passivity layer) especially in reducers environment and mineral acids. Stabilizer for Austenit. In Cr-Ni alloys, the opposite effects of both are combined to produce alloys with an extended range of structures and properties.

Mn: In moderate quantities gives to the alloy the same effects of Ni, although the change of Ni for Mn is not practical. O Mn combines itself with the S, to turn into Mn2SO4 and make it more plastic at high temperatures.

Mo: In combination with the Cr, is effective for stabilizing the passivity layer in chlorates presence, increasing the resistance of the alloy to corrosion gaps and clefts. Ferrite stabilizer. In martensitic steel, combines with carbon and reduces the temper, needing the use of higher austenitization temperatures. Increases the resistance of austenitic steel in high temperature.

C: Element that gives the temper by thermal treatment of martensitic steels, besides it promotes the mechanical resistance in applications at high temperatures. In other applications, the C is harmful to corrosion resistance, because of its combination with Cr. In ferritic steel, the increase of amount of C causes the break of tenacity.

N: Increases the mechanical resistance and pites corrosion resistance in austenitic steels. But the N is harmful for mechanical properties of ferritic steels. It has a similar effect to carbon, stabilizer for austenit , used to mantain the austenitic structure in steels which nickel amount is reduced to do so with costs.

Si: Makes better the resistance of the alloys to oxigen, air or oxide gas attack. Primary addition of heatproof alloys. Stabilizer for Ferritic.

Ti e Nb: Presents high fidelity with carbon and so, avoid the precipitation of chromium carbonites during slow cooling or long staying in temperatures in order of 700ºC, avoiding the local loss of chrome, which means a disaster for corrosion resistance. They are strong ferrite stabilizers. In austenitic steels, they improve the hot-in resistance, through carbon nitrates precipitation in austenitic matrix.

Austenitic Stainless Steel

 

Not hardenable by rapid cold from high temperature, but hardened when cold.

Due to its good stainless features, it's very used in parts that need resistance to corrosion or in chemical equipment.

Used also as heat resistant, due to its good resistance to oxidation and softening at high temperatures.

Requires attention in what is about excessive heat, due to the non-refining of the grain by thermal treatment.

In solution conditions, the majority of the steels are not magnetic, but in cool metalworking beyond the increase on hardening, it gives a slight magnetic sensitivity.

In AISI 304, when heated over 600ºC, it might occur corrosion all over the grain. So, for these applications, we suggest the AFP 304L, AFP 316L - low carbon amount.

Ferritic Stainless Steel

 

Is not hardened by temper (martensitic transformation).

 Are even more stainless than martensitic stainless steel, in oxide solutions or  atmospheric environments.

 As it doesn't have refining of the grain with thermal treatment, it's necessary to  be careful about high temperatures re heating.

 The high chrome stainless steel exposed for a long time at 500ºC happens to  become fragile, so it's very important to choose the parts to be used.

 In any condition, as the majority of its structure is ferritic (soft), it has good  capacity to cold shaping.

 Performs good magnetic sensitivity.

Martensitic Stainless Steel

 

Hardening at rapid cooling from higher temperatures (martensitic transformation). The temperature reduction gives the possibility to obtain a large range of hardness, resistance, deformation and tenacity.

Good resistance to oxidation in atmosphere without losing hardness until 500oC, able to be used as heat resistant.

Good resistance to solutions as nitric acid in environmental temperature, but corrosive in reducing solutions as sulfuric acid and chloridric acid. The resistance decreases with the raise of amout of elements as Carbon, S, Ph.

Needs attention when solding, because it tends to wreck, due to its resistance obtained in temper.

No matter if re cooked, or tempered, present magnetic sensitivity.