FAQ - Technical matters
This is a list of frequently asked questions about doing business with Tokkin.
If you have a question about a topic that is not covered here, please don't hesitate to contact us.
- Is stainless steel magnetic?
Some stainless steels are magnetic, but others are not.
This depends on the type of stainless steel.
It is said that cheap stainless steels stick to a magnet.
People have this image because non-magnetic stainless steel is austenitic stainless steels containing expensive Ni.
*The price of stainless steel is decided not only by its component, so please be aware that in making a plan.
Ferritic stainless steels typified by SUS430 are magnetic.
Our company have SUS430、FS-1*、TDS-1*.
TDS-1 is used for magnetic shielding, cores etc. as an electro- magnetic stainless steel.
*They are our original steel types.
Austenitic stainless steels typified by SUS304 are generally nonmagnetic.
However, SUS301 and SUS304 are transformed to magnetic state because of structural transformation caused by cold working. So, you need to be aware that in processing.
Oppositely, as for SUS305, SUS316L, SUS310S, etc., Structural transformation is not likely to happen. So, they maintain non-magnetic state even after processing.
(It differs in degree depending on the type of steel, please contact us for further information.)
When you need non-magnetic stainless steel, we recommend you austenitic stainless steels.
However, depending on strength, application and processing way, you need to select the most appropriate type of stainless steel.
When you have any questions in selecting type of steels, feel free to contact us.
*We recommend you the most appropriate steel type even from other than our product lineup depending on your required quality, budget, etc.
- Why various sizes of rollers are used for a rolling mill machine?
When rolling is done by one roller, the roller changes shape by the pressure.
Rolling materials with a deformed roller makes defective products, so in order to prevent deforming rollers it is necessary to use various sizes of rollers to disperse the force.
- What is corrosion resistance? | Corrosion Resistance of Stainless Steel
What is corrosion resistance?
Corrosion resistance is the ability of a material to resist corroision coused by oxidation or chemical reaction.
Corrosion resistance of stainless steel
Stainless steel is know as a metal which is less prone to rust and this is bacause of the protective action of the film (passive film) existing on its surface.The main component of this film is an oxide (hydroxide) containing chromium, which is closely bonded to the metal to protect the metal. Therefore, in an environment where this film is not damaged, Corrosion Resistance is maintained for a long time.
For example, even if the film is broken in the air, it will be automatically regenerated by
oxygen in the air. This is why it is strong against oxidizing acids such as nitric acid and concentrated sulfuric acid.
On the other hand, in a reducing environment, destruction and regeneration of the film are blocked, so corrosion occurs relatively easily.To improve this, Elements such as nickel, molybden, copper are added to some steel grades to make its passive film strong.
In addition, when foreign matter may adhere to the surface, contact with dissimilar metals, or
there is a temperature or concentration gradient, local corrosion may occur. The surface of stainless steel should be as smooth and clean as possible, bacause corrosion tends to occur especially when the surface is rough or dirty.
Generally, when the oxidizing property of the environment becomes strong, metal corrosion is likely to occur, but for
some metals, when oxidizing properties get stronger than a certain level, corrosion may stop.
Such situation is called “Passivation” of the metal, which is caused by the combination of the metal and the environment.In addition, some metals are easily passivated, such as Fe, Ni, Cr, Mo and their alloys (eg: stainless steel).
The following are possible methods for promoting passivation.
(1) Method of immersing in a solution containing nitric acid or other strong oxidants
(2) Method of anodizing a material in a solution includiong an oxidizing agent.
(3) Method of heating materials with low temperature in clean air or oxygen.
Of these, the mainly used method is(1) and there are following methods.
(a) 50% boiling HNO3
(b) 20-40% 60°C HNO3
(c) dipping into 4% HF + 4% CrO 360°C
All for 30-60 minutes.
Also, it is said that the effect is enhanced if the surface is activated before the passivation treatment.
In any case, the thickness of the passivation film is very thin, from several tens of Å to 200 Å, so there may be
problems with the permanent effect depending on the environment.
When austenitic stainless steel is heated to around 400-800°C, carbide precipitates at the grain boundaries and corrosionresistance deteriorates significantly. This is called Sensitization, this is caused by keeping in this temperature range or by
slowly cooling, especially in the heat affected zone during welding.
The following methods are used to prevent this.
(1) Uses low carbon steel with a particularly low carbon content（SUS304L、316L、316J1L）
(2) Add carbide forming elements such as titanium and niobium（SUS321、347）
Reheat the precipitate of carbide to 1000-1150℃ and Cool down rapidly(solution heat treatment)
we recommend the most appropriate type of stainless steel depending on the cause of corrosion.
Also, We might recommend other steel other than stainless steel depending on usage environment and other your request.
If you have any trouble with rust or corrosion, feel free to contact us by an inquiry form.
- What is the Finish (condition) ?
The metal materials can change a mechanical characteristic by rolling or heat-treatment.
Please appont finish if you want to buy materials which appointed strength.
*1 B= Bright finish *2 D= Dull finish
JIS G 3141
Full Hardness SUS301
JIS G 4313
JIS G 4305
1/4H Rolled 1/2H Rolled 3/4H Rolled H Rolled EH Rolled SEH Rolled Other
JIS G 4305
R Rolled Special steel A Annealed
JIS G 4802
R Rolled Quenching and Tempering Quenching and Tempering treating