1. What is the influence of Sulphur in the machining ability of the stainless steel?

In a similar way that in steel for mechanical construction, innumerable methods for the improvement of the machining ability of stainless steel exist. The method more known and used is the addition of sulphur to the steel, which leads the formation of manganese sulfates in the matrix. These inclusions possess low point of fusing and high folding ability, having a positive effect in the machining ability. They cause a lubricate effect in the cut edge and facilitate the cavaco in addition in the shear zone. Other methods use the addition of elements as selenium, lead, tellurium and bismuth. However, most of the time, the improvement of the machining ability is followed by the fall of other properties. In particular, the following properties can be harmed:
- Resistance to the corrosion.
- Ductility and tenacity in the transversal direction.
- Plasticity to hot and cold.
- Weld ability.


2. Does the addiction of Bismuth in the stainless steel increase its machining ability? If it increases why/how does it occurs?

The bismuth (Bi) addition significantly increases the machining ability of austenitic stainless steel. Normally the stainless steel of free cut contains sulphur (S), as in the steel type 303. However sulphur causes a deterioration of the corrosion resistance, restricting its application to non-aggressive environments, beyond being able to modify the flavor of foods and drinks due to sulphur. An alternative is the use of heavy metals of low fusing point, as the lead (Pb), as additive of machining ability improvement. However, due to ambient and health problems the use of the lead addition, especially in applications in the food industry, must be prevented. Considering that the Bi is one of the safer heavy elements, as confirmed for its use in cosmetics and medicine, its use as machining ability addition was made in some types of steel, also in the stainless steel.

The machining ability of one steel type 304 I containing Bi is comparable of one type 303 steel, with 0,17% S. The particles of Bi act in similar way to particles of lead, meaning, they lubricate the interface of the tool and the cavaco, reducing the attrition. Additionally they act as fragileness and they help in the cavaco cleavage. The stainless steel with addition of Bi, presents machining ability equivalent to the stainless steel type 303, but with mechanical properties and of corrosion equivalents to type 304.

Many technological properties are kept unchanged, as weld ability, work ability in the cold, etc. Only the work ability in the hot one is reduced, needing well-taken care of special in relation to the 304, as band of recommendable temperature of forging between 1100 and 1200 ºC. In view of that the Bi is a much more expensive addition of what the S, these steel has higher price.
For better information we suggest to read to the article: "A Usinabilidade do Aço Inoxidável" Authors: Marcelo B. Tessler and Celso. Barbosa, published in the magazine Metallurgy and Materials of the AMB, Vol. 49, nº 413, January 1993, pp. 032-041.


3. Which are the main chemical proprieties that determine the non-magnetization capacity of the stainless steel?

In a general way, what determines magnetic proprieties to a stainless steel is the chemical balance of elements in its composition, therefore through it determinates the austenitic or ferritic structure.

The ferritic stainless steel are magnetic and austenitic the non-magnetic ones. However the austenitic steel when conformed can have the austenitic of its structure transformed into Martensitic, conferring magnetic proprieties to the material.


4. Stainless steel inhibitors in corrosion in sulphuric acid.

The stainless steel normally is not used in sulfuric acid. In diluted concentrations (less than 25%) or very concentrated instead, 316L is resistant since that the system works in low temperatures (ambient temperature).

The 904L stainless steel resists in all concentrations but there are limitations with the temperature. It is not much usual to use inhibitors to be able to use stainless steel in a sulfuric acid. The sulfuric acid and the ferric sulphate added allow the stainless steel behave in a passive form in acid. But the ratio of ferric sulphate to be added depends on the concentration of acid.

In diluted solutions of sulfuric acid and with low temperatures, the oxygen helps to resist better the corrosion. In this case can be injected air by pressure. But these cases are very specific ones, with low concentrations of sulfuric acid. 


5. Can the stainless steel be used in the process equipment confection that works with chloridric acid 20% in room temperature? And what about the ascetic acid 15% in a temperature of 60ºC?

a) No stainless steel is recommended to work in chloride environments. The ambient temperature and with concentration 20% (and also with lesser concentrations) the stainless steel do not resist the chloridric acid.

b) With temperature of 60 degrees Celsius and concentration of 15%, the steel 304L resists acid the ascetic one. Cares must be taken to prevent temperature increases (the 80 degrees Celsius the same solution would attack 304L, but it would not attack 316L).


6. I would like to know which is the formula used with nitric acid to recompose the passive layer of the stainless steel.

To recompose the passive layer it is recommended to use an acid solution of nitric (15% or 20%) that it must act on the material per 30 minutes, after what, the same must be washed with abundant water.


7. Which is the effect of sulphur in the machining ability of the stainless steel?

In a similar way that in steel for construction mechanics, innumerable methods for the improvement of the machining ability of stainless steel exist. The method more known and used is the addition of sulphur to the steel, that leads the formation of sulphetes of manganese in the matrix. These inclusions possess low point of fusing and high deformability, having a positive effect in the factoring. They cause a lubricate effect in the cut edge and facilitate the cavaco in addition in the shear zone.


8. Chemical corrosion in stainless steel to produce plates in low and high relief.

Normally, a covering in the part is made that does not have to be attacked.
Later, the part it is placed in a ferric chloride solution (approximately 10 20%) that it will attack the part that in we intend them either attacked. Solutions of acid nitric and hydrofoil acid also can be used. Increasing the temperature of the solutions can be obtained increase in the speed of the attack. The necessary time must be established by you, in function of the depth of desired attack.


9. Inhibitors of the corrosion of the stainless steel in sulfuric acid.

The stainless steels normally are not used in sulfuric acid. In diluted concentrations (less than 25%) or very intent, 316L is resistant since that the system works in low temperatures (ambient temperature). The stainless steel 904L, that it is not manufactured in Brazil, resists in all the concentrations, but limitations with the temperature exist.
It is not very usual to appeal inhibitors to be able to use a sulfuric acid stainless steel in. The sulfuric acid ferric sulphate added allows that the stainless steel if holds of passive form in this acid one. But the ratio of ferric sulphate to be added depends on the concentration of the acid one.

In solutions diluted of sulfuric acid and with low temperatures, the oxygen helps to resist the corrosion better. In this in case that air can be injected the pressure. But we are saying of cases many specific ones with sulfuric acid of low concentration.


10. Which are the acids that corrode the stainless steel?

We can say that the acid reducing (as the chloridric and the hydrofoil acid) attack all stainless steel. Acid oxidants do not attack as nitric acid does. Sulfuric acid attacks stainless steel in practically all the concentrations and temperatures. The stainless steel only resists this acid one in solutions very diluted or very concentrated and in ambient temperatures. The stainless steel resists the acid phosphoric in good band of concentrations well.


11. I would like to know which are the electrochemical experiments used to compare steel, and which one resists more to the corrosion.

An only test does not exist to compare the resistance with the corrosion of diverse types of steel, since this are affected by the way of work, temperature, concentrations half them, etc.
They exist however, studies that allow to more recommending the type of adjusted steel in resistance terms to the corrosion, in function of the conditions mentioned above.


12. Which acids and which concentrations can we use to scour and to passive screws in AISI 420 and 410 that had been tempered and annealing?

For pickling and passivity of tempered steel 420 and 410 we recommend 15% HNO3 + 0.5% HF the temperatures between 50 and 60 degrees C; But for passivity we recommend 20% HNO3 + 2% In 2Cr2O7.2H2O (sodium dichromate) or optionally 50% HNO3, in both the cases also preferential between 50 and 60 degrees C.

The time must be determined for each case in particular through the evaluation of samples treated from 5 minutes (minimum time). The time depends on the concentration and temperature of the bath and the result that if it desires to get.


13. Which are the steel and thickness recommended to manufacture a 700x700x1000mm tank one where I will use acid moriatic and caustic soda water in high temperature, in order not to be a deformed tank and does not have corrosion?

The stainless steel materials are not recommended for use in presence of chloridric acid. In any concentration and temperature the chloridric acid attacks all stainless steel.


14. Which is the influence of CHROMIUM and NICKEL in the stainless steel?

We can define as stainless steel the group of leagues of iron (Fe) and chromium (Cr) I contend 10.5% of Cr at least. Other metallic elements also integrate are leagues, but the Cr is considered the element most important because it is what it gives to stainless steel one raised resistance to the corrosion.

CHROMIUM:
Exactly when the steel inox suffers some type of damage, they are scratches, kneading or cuts, immediately the oxygen of air agrees with chromium, forming again the protective film, resetting the resistance to the corrosion. This quality is inherent to the steel inox since the chromium is part of its chemical composition.

NICKEL, added in the stainless steel is responsible for:

Increase the resistance to the corrosion in a general way.
Improve the Ductility (Stamping).
The hot one increases the Resistance mechanics.
Improve the Weld ability.


15. For manufacturing tanks for water storage with average temperature of 60°C, which is the chemical composition of the water that would be allowed for stainless steel 304 and 316 not to occur corrosion in these tanks?

For the 304 stainless steel the maximum ion text chloride (CL) in the water the temperature of 60°C, is of 0,015% and for steel 316 it is of 0,08%.


16. How to distinguish the 304 stainless steel from the 316 by chemical ways and where can I find the product to purchase it?

To distinguish steel 304 and 316 is necessary to identify the presence of Molybdenum, present only in type 316. 16® has in the market a called product DETET, developed to effect the qualitative analysis of stainless steel AISI 316, through the molybdenum; Ideal for the industries of connection, mechanical and metallurgic in general.