FAQ´s

What is stainless steel?

The stainless steel is essentially a low carbon steel, which has a minimum chromium content of approximately 10.5% by weight, what makes it a resistant material to the corrosion.

What is corrosion?

The corrosion is the deterioration of a material due to the electrochemical interaction with its environment; it is the spontaneous mechanism, with which the nature reverts the processes of obtaining of the materials causing the alteration or destruction of the most of the goods manufactured by the man. One of the most evident corrosion manifestations is the rust.

Why is stainless steel resistant to the corrosion?

The contained chromium in the alloy combines with the air oxygen to form a thin transparent film of chromium oxide on the steel surface. The film is named passive layer and it confers to the stainless steel its resistance to the corrosion. The passive film of chromium oxide is coherent, homogeneous and impermeable; if mechanical damage occurs, this film self-regenerates in oxygen presence.

How are stainless steels classified?

The stainless steel can be classified in five different families; four of them correspond to the particular crystalline structures formed in the alloy: austenitics, ferritics, martensitics and duplex (austenite plus ferrite); while the fifth are the precipitation-hardening alloys, which are based on more in the type of used heat treatment than in the crystalline structure.

The first two families are available commercially in Mexico, as well as bars and solepieces in martensitics, while the duplex families and the precipitation-hardening alloys are only obtained by the import.

What kind of stainless steel is most utilized?

The most common stainless steels are austenitics, basically because they offer excellent resistance to the corrosion, good formability and weldability.

Which are the characteristics of the austenitic stainless steels?

The austenitics are obtained adding austenite former elements, such as nickel, manganese and nitrogen. The chromium content generally varies from 16 to 26% and its content of carbon is of range from 0.03 to 0.15%.

The austenitics have excellent resistance to the corrosion, weldability, have the ability to be functional in extreme temperatures, they are of simple-shaped and easy transformation; they offer an excellent factor of hygiene and cleaning. They are hardened by cold work and not for heat treatment, they are not magnetic in their annealing condition, and however they acquire certain magnetism due to the cold work.

CHEMICAL COMPOSITION OF THE AUSTENITICS STAINLESS STEELS

Type
AISI

Designation
UNS

C

Cr

Ni

Mn

Si

Others

201

S20100

0.15

16.0 - 18.0

3.5 - 5.5

5.5 - 7.5

1.00

0.25%% N

301

S30100

0.15

16.0 - 18.0

6.0 - 8.0

2.00

1.00

----

303

S30300

0.15

17.0 - 19.0

8.0 - 10.0

2.00

1.00

0.6%% Mo

303Se

S30323

0.15

17.0 - 19.0

8.0 - 10.0

2.00

1.00

0.15%% min Se

304

S30400

0.08

18.0 - 20.0

8.0 - 10.5

2.00

1.00

----

304L

S30403

0.03

18.0 - 20.0

8.0 - 12.0

2.00

1.00

----

308

S30800

0.08

19.0 - 21.0

10.0 - 12.0

2.00

1.00

----

309

S30900

0.20

22.0 - 24.0

12.0 - 15.0

2.00

1.00

----

309S

S30908

0.08

22.0 - 24.0

12.0 - 15.0

2.00

1.00

----

310

S31000

0.25

24.0 - 26.0

19.0 - 22.0

2.00

1.00

----

310S

S31008

0.08

24.0 - 26.0

19.0 - 22.0

2.00

1.00

----

316

S31600

0.08

16.0 - 18.0

10.0 - 14.0

2.00

1.00

2.0 - 3.0 %% Mo

316L

S31603

0.03

16.0 - 18.0

10.0 - 14.0

2.00

1.00

2.0 - 3.0 %% Mo

317

S31700

0.08

18.0 - 20.0

11.0 - 15.0

2.00

1.00

3.0 - 4.0 %% Mo

317L

S31703

0.03

18.0 - 20.0

11.0 - 15.0

2.00

1.00

3.0 - 4.0 %% Mo

321

S32100

0.08

17.0 - 19.0

9.0 - 12.0

2.00

1.00

5 X %%C min Ti

347

S34700

0.08

17.0 - 19.0

9.0 - 13.0

2.00

1.00

10 X %%C min Nb + Ti

Which are the characteristics of the ferritic stainless steels?

The ferritics are essentially alloys with chromium. The chromium content is usually from 10.5 to 30%, but limited contents of carbon of the order of 0.12%.

These stainless steels of the 400 series maintain a stable ferritic structure from the ambient temperature up to the melting point. They offer resistance to the corrosion from moderate to good, which is increased with the chromium content and molybdenum additions. They cannot be hardened by heat treatment, are magnetics, their weldability is poor therefore the unions by welding are generally limited to thin gauges. An annealing treatment generally is applied to them with what obtain bigger softness, ductility and resistance to the corrosion. Due to their poor hardness, the use is generally limited to processes of cold work.

CHEMICAL COMPOSITION OF THE FERRITICS STAINLESS STEELS

Type
AISI

Designation
UNS

C

Cr

Ni

Mn

Si

Others

409

S40900

0.08

10.5 - 11.75

---

1.00

1.00

Ti (6 X %%C) min.,   0.75 max

430

S43000

0.12

16.0 - 18.0

---

1.00

1.00

----

430F

S43020

0.12

16.0 - 18.0

---

1.25

1.00

0.15 %%S min, 0.6 %%Mo

430FSe

S43023

0.12

16.0 - 18.0

---

1.25

1.00

0.06 %%S, 0.15 %%Se min

434

S43400

0.12

16.0 - 18.0

---

1.00

1.00

0.75 - 1.25 %% Mo

439

S4300

0.03

17.0 – 19,0

---

1,00

1,00

Ti [0.20+4(C+N)] min, 1.10 max; Al 0,15.

441

S4400

0.03

17.5 – 18.5

---

1.0

1.0

Nb [0.30 + 3( %%C)] min.,  1,0 max.
0,10 – 0.60 Ti

Which are the characteristics of the martensitic stainless steels?

The martensitics are essentially chromium and carbon alloys. The chromium content is generally from 10.5 to 18% and the carbon is high, reaching values up to 1.2%. They are the first branch of the stainless steels simply to the chromium. They represent a portion of the 400 series. The martensitic stainless steels offer a moderate resistance to the corrosion, are hardenable by heat treatment and therefore high levels of mechanical resistance and hardness can be developed. They are magnetics and, due to the high content of carbon and to the nature of their hardness, they have poor weldability.

CHEMICAL COMPOSITION OF THE MARTENSITICS STAINLESS STEELS

Type
AISI

Designation
UNS

 

C

 

Cr

 

Ni

 

Mn

 

Si

 

Others

410

S41000

0.15

11.5 - 13.0

----

1.00

1.00

----

420

S42000

0.15 min

12.0 - 14.0

----

1.00

1.00

----

431

S43100

0.20

15.0 - 17.0

1.25 - 2.50

1.00

1.00

----

440A

S44002

0.60 - 0.75

16.0 - 18.0

----

1.00

1.00

0.75 Mo

440B

S44003

0.75 - 0.95

16.0 - 18.0

----

1.00

1.00

0.75 Mo

440C

S44004

0.95 - 1.20

16.0 - 18.0

----

1.00

1.00

0.75 Mo

Which are the characteristics of the duplex stainless steels?

They are alloys chromium-nickel-molybdenum; they have content between 18 and 26% of chromium and 4.5 to 6.5% of nickel.

The duplex are magnetics, they cannot be hardened by heat treatments, and they offer good weldability. The duplex structure improves the resistance to the corrosion of fracture under tension in environments with chlorine ions.

CHEMICAL COMPOSITION OF THE DUPLEX STAINLESS STEELS

Type
AISI

Commercial
Name

Cr

Ni

Mo

N

Others

S32304

SAF 2304        SR 35 N

23

4

0.2

0.1

 

S32404

UR 50              VEW A903

21 18.5

7  5

2.5 2.7

0.07   0.07

1.5 Cu               1.5 Si

S31803

UR 45N         SAF 2205

22

5.3

3

0.16

 

S31260

DP-3

25

6.5

3

0.16

0.5 Cu           0.3W

S32550

UR 52 N

25

6.5

3

0.18

1.6 Cu

S32200

VEW A905

26

3.7

2.3

0.34

6 Mn

S32760

ZERON 100

25

7

3.6

0.25

0.7 Cu               0.7 W

S32550

UR 52N+

25

6

3.8

0.26

1.5 Cu               1.5 Si

Which are the characteristics of the precipitation-hardenable stainless steels?

This family offers an alternative to the austenitic stainless steels when you want to associate high mechanical and machinability characteristics. They are alloys iron-chromium-nickel that are characterized by the mechanical resistance obtained starting from the hardening by aging heat treatment. The precipitation-hardenable steels are patented and frequently they are designated with the initials of the producer company.

CHEMICAL COMPOSITION OF THE PRECIPITATION-HARDENABLE STAINLESS STEELS

Type
AISI

Commercial
Name

C

Cr

Ni

Others

635

Stainless W

0.07

16.5

6.8

0.5 Mn/ 0.5 Si/ 8Ti

630

17-4 PH

0.04

16

4

0.25 Mn/ 0.6 Si/ 3.2 Cu/ 0.25 (Cb+Ta)

****

15-5 PH

0.07

15

4

4.0 Cu/ 0.35 (Cb+Ta)

SEMI AUSTENÍTICOS

631

17-7 PH

0.07

17

7

0.6 Mn/ 1.15 Al

632

PH 15-7 Mo

0.07

15

7

2.20 Mo/1.15 Al

633

AM-350

0.10

16.5

4.3

2.75 Mo/ 0.10 N

600

A286

0.08

15

26

1.3 Mo/ 0.3 V/ 2.0 Ti/ 0.35 Al/ 0.003 B/ 0.4 Si

****

17-10 P

0.12

17

10

0.75 Mn/ 0.6 Si/ 0.25 P

****

17-14 CuMo

0.12

16

14

3.0 Cu/ 2.5 Mo/ 0.5 Cb

Which is the meaning of subindex "L" in some types of stainless steel?

It means that it is a variant of the normal grades whose content of maximum carbon is limited to 0.03%, with what the risk of a possible sensitization is minimized and is prone to suffering intergranular corrosion.

Which is the utility of the contained nickel in some alloys?

The nickel is an element called "gammagene" or promoter of the formation of the austenite phase. The above-mentioned means that the nickel addition, in this case to the stainless steels, allows to obtain the austenitic phase (it is present and stable to temperatures above 923° C) to temperatures inclusive below 0° C. The main function of the nickel in the stainless steels is the improvement of the formability and ductility. It also increases until certain grade the material hardness and in certain way it influences a little in the increment from the resistance to the corrosion.

Which is the utility of the contained molybdenum in some alloys?

The contained molybdenum in some alloys increases the resistance to the corrosion, particularly in sour and marine environments. The molybdenum increases the resistance to the corrosion for pittings and cavities.

Which is the utility of the contained nitrogen in some alloys?

It improves the resistance to the corrosion for cavities and it offers a good combination of hardness and weldability.

Can stainless steel be used in extreme temperatures?

Yes. The stainless steel has an excellent performance in very high temperatures (up to 925° C) and under cryogenic conditions.

Can stainless steel be welded?

Yes, you can weld it easily.

Is the stainless steel welding similar to that of the carbon steel?

The stainless steel welding is made by nature different from the carbon steel welding and from the steels of low alloy as for its operation parameters. The stainless steel requires of processes that, inside its parameters, includes protection of the welding puddle in order to prevent it from oxidizing. Their electric resistance is much bigger than the carbon steel and therefore the welding processes require smaller current intensity.

What types of finishes can we give to the stainless steel?

FINISHES BY LAMINATION: They are carried out in the heat and cold lamination processes; they are basically supplied in all the flat products of stainless steel. It is enough for some applications, but they are also the base for other processes of surface modification.

GRINDING AND GLOSSY MECHANICAL FINISHES: They are obtained after applying a series of operations of grinding and polished with different sizes of abrasive grain. (The most common finishes are the P3, P4 and the popular mirror finish)

Finish

Appearance

Grain

Roughness RMS
RMS

Roughness RMS
RA

#1

Rough

N/A

N/A

N/A

2D

Matte

N/A

N/A

N/A

2B

Bright

N/A

N/A

N/A

#3

Scratch brushed

100

30-45

25-40

#4

Brushed

120-180

18-30

15-25

#6

Satined

240

14-20

12-18

#7

Polished

320

5-9

4-8

#8

Mirror

400

1-5

0-4

ENGRAVINGS FINISHES: They are obtained by laminating the sheet of stainless steel with rollers previously engravings with drawings. This process superficially hardens the plate which allows the use of materials with thinner thickness, with the consequent saving and reduction of the total weight.

SANDBLASTED FINISH BY SAND: the peening with sand provides uniformity, a non directional surface, of low reflectivity that contrasts well, visually, with the well polished finishes.

ELECTRO-POLISHED: This chemical process is carried out in sheets as well as in finished components. It is used to achieve a surface of the flatter and reflective material eliminating the "peaks and valleys" of the irregular surface.

COLORED FINISHES: Can give color to the layer by means of a chemical process, hardened by an electrolytic process. The austenitic steel is the most appropriate for this process. Depending on the time, during the immersion of the steel in an acid solution, the layer is formed in the surface and by means of the physical effect of the light interference, that is, the overlapping of the light that enters and is reflected, it is produced an effect of intense color.

ENGRAVING WITH ACID: It is a process that eliminates a part of the surface material. The engraved surfaces have a matte, wrinkled appearance and it contrasts with the surfaces with polished or satiny finishes.

Does the finish of the stainless steel have some relationship with its resistance to the corrosion?

Yes, different types of finishes for the same type of stainless steel can lead to different behaviors in certain aggressive environments. In general, if the superficial finish is less rough, the resistance to the corrosion of material will be bigger.

The resulting finishes of abrasive processes have lesser resistance than the obtained for the manufacturing traditional processes (lamination, annealing and final pickling).

Why is it said that the stainless steel has a smaller cost of life cycle than other materials?

If we consider the initial investment, the cost of cleaning, maintenance and replacement of parts of a stainless steel application in their useful life, it is less expensive than other materials that require maintenance or replacement, still when the initial investment has been bigger. The frequency and cost of cleaning and maintenance of the stainless steel is smaller than other materials, for this reason it is said that even then the initial investment is bigger it has a smaller cost in its life cycle.

Is stainless steel easy to clean?

Regularly it is enough to clean with water and soap, to rinse with clean water and to dry off with a cloth.

How often should I clean the applications of stainless steel?

Generally 3 or 4 times per annum if they are outside and every day if hygienic conditions are required (kitchens, hospitals, pharmaceutical industry, etc.) or aggressive conditions prevail (saline environment, pollution, humidity, chemicals, etc.), apart from these cases it should be cleaned when it is dirty so that it recovers their original appearance.

Why is it said that the stainless steel is anti-vandal?

Because the graffiti and the paint can be cleaned with alkaline solvents or paint solvents recovering the original appearance of the material.

Which are the ideal conditions to use stainless steel?

When it is presented in an individual form and with more reason, some of the following factors combined: Corrosive environments, very low temperatures (cryogenic), high temperatures, when high mechanical resistance is required vs. little mass, hygiene, aesthetic appearance, durability, and little maintenance.

Why are the kitchen utensils of stainless steel better than the fabricated ones in copper, aluminum, spelter or other materials?

The stainless steel does not chip off, is not toxic, neither alters the flavor or scent of the food. The flat surface and low porosity of the stainless steel makes the cleaning process easier and it diminishes the bacteria proliferation.

Why is the stainless steel the ideal material to manufacture utensils and kitchen parts?

Besides their beauty and durability, the stainless steel is quick and easy to clean. It doesn't absorb the fat, neither other foods, it is resistant to temperature changes and blows.

It has an excellent appearance when is combined with materials as the wood, marble, glass and/or tempering glass.

Why is it used in the production of dishwasher, washing and drying machines components?

Basically because the constant use of detergents, soaps and chlorine as well as high temperatures could damage the surfaces of the tackle interiors, if they were manufactured in synthetic materials or with covered protectors. The stainless steel fulfills the conditions that these apparatuses demand.

Why is the stainless steel recommended for the fabrication of implants?

Because is a biomaterial. It is not toxic neither carcinogenic and cohabits with the human tissues and fluids without causing reactions. It has an appropriate mechanical resistance, weight and fatigue time. It is cheaper than other materials employed in the production of implants as the titanium.

Why is stainless steel the most utilized material in the medical, pharmaceutical and nutritional industry?

Because their non-porous surface avoids those germs, bacteria or dirt are harbored and its easiness of cleaning allows you to obtain conditions of hygiene and asepsis. It is resistant to extreme temperatures, cleaning with vapors, disinfectants and germicides, it does not flake neither it wears away.

Why is stainless steel the ideal material in the manufacture of train boxcars and buses?

Because it absorbs energy 2.5 more times than the carbon steel, this allows it a lesser deformation in collisions. The stainless steel is resistant to the fire and offers best properties than other materials, also, the vehicles elaborated in stainless steel have a lesser weight since we can obtain similar mechanical properties to those from the carbon steel, using materials of thinner thickness, which entails savings in fuel and maintenance of the roads. It offers a long life and minimum maintenance.

Why is it said that the stainless steel is an ecological material?

Because all the stainless steel products are 100% recyclable

How is it possible to differentiate stainless steel from carbon steel, in field and in quick way?

Video (Spanish)

To detect the difference between stainless steel of any type and carbon steel we have the non-destructive testing of the copper sulfate.


The test consists basically on adding a drop of a solution of copper sulfate on the material surface that you want to verify. If inside the drop you can appreciate the copper precipitation or watch a copper coloration, the material which is observed is a carbon steel. If the drop doesn't present any alteration and it remains such as it is (initial color), the analyzed material is a stainless steel.


This method is only useful to distinguish between a stainless steel and carbon steel; however, it is not good to differentiate between types of steels, that is to say, it doesn't distinguish whether to the analyzed steel corresponds to a ferritic, austenitic or martensitic stainless steel.


The solution is prepared in the following way:

To prepare 100 ml of the solution:

Separately must measure 10 ml approx. of 95% sulfuric acid

To weigh 10g of copper sulfate reactive grade (98.8%).

To gauge to 100 ml with distilled water


Firstly it is added the sulfuric acid slowly to the distilled water, trying that the acid slips for the recipient walls, for this you can bend just a little the recipient to achieve this effect (it must always be done in this way since if we empty the water to the acid it happens an exothermic reaction and the solution will be shot out being able to cause damage to the person that is preparing it)


For security it is recommended to use protection for the eyes and nose since the vapors given off by the sulfuric acid are very irritating and dangerous. In case of contact with the solution in an accidental way, it is necessary to be rinsed with abundant water and soap to neutralize the action of the solution.


After the acid has been added to the water in a slow and careful way, this solution must be shaken. Then the copper sulfate is added and shaken once again to homogenize the solution. After this, the solution is ready to be used. You can apply the solution with a dropper.

How is detected in field if a stainless steel contains molybdenum?

Video (Spanish)

Through the simple non-destructive method to identify austenitics steels with molybdenum, which consists on connecting the positive pole of a battery to the material that is going to be analyzed; the negative pole of the same battery is connected to a graphite electrode (it can be the graphite of a pencil tip or a battery center).
On the surface of the material to be analyzed a drop of an electrolytic solution is placed, then the graphite electrode is put in direct contact with the solution drop for a moment, between two and five seconds, you must be careful in order to the electrode doesn't touch the steel. Now it is observed that the solution becomes rosy or red color.
If the material contains molybdenum, when the graphite electrode of the drop is withdrawn, it will remain in pink or red color. If the solution drop becomes green, rose, red or transparent and when the graphite electrode is withdrawn the coloration disappears, it indicates that the material doesn't contain molybdenum.
The used electrolytic solution consists of three parts of potassium thiocynate (KCNS) and a part of stannous chloride (SnCl2) prepared when dissolving 25 g of SnCl2 in 20 ml hydrochloric acid (HCl) and diluting to 100ml with water.
This is:
If 100 ml of electrolytic solution are going to be prepared, it will be in the following method:
75 ml of KCNS and 25 ml of SnCl2.
The 25 ml of SnCl2 will be prepared in the following mode: 6.25g of SnCl2, 5 ml of HCl and add water until you have 25 ml solution.
Later both components must be blended and proceed with the test.
If you wish to prepare a smaller quantity of electrolytic solution, the proportions must be respected.
This solution can remain chemically active for a period of two to three months but it should be periodically verified using the solution with material 316 and 304 and the above-mentioned behavior should be observed. If different behaviors are not observed assuring that the materials are correct, then the solution has lost its power and a new solution must be prepared.