Technical Information

Can stainless steel use at low and high temperatures?

Austenitic stainless steels are extensively used for service down to as low as liquid helium temperature (-269 deg C).
This is largely due to the lack of a clearly defined transition from ductile to brittle fracture in impact toughness testing. Toughness is measured by impacting a small sample with a swinging hammer. The distance which the hammer swings after impact is a measure of the toughness. The shorter the distance, the tougher the steel as the energy of the hammer is absorbed by the sample. Toughness is measured in Joules (J). Minimum values of toughness are specified for different applications. A value of 40 J is regarded as reasonable for most service conditions.
Steels with ferritic or martensitic structures show a sudden change from ductile (safe) to brittle (unsafe) fracture over a small temperature difference. Even the best of these steels show this behaviour at temperatures higher than -100 deg C and in many cases only just below zero.
In contrast austenitic steels only show a gradual fall in the impact toughness value and are still well above 100 J at -196 deg C. Another factor in affecting the choice of steel at low temperature is the ability to resist transformation from austenite to martensite.

Can I use stainless steel at high temperatures?

Cleaning methods for stainless steel

Stainless steel is easy to clean. Washing with soap or mild detergent and warm water followed by a clear water rinse is usually quite adequate for domestic and architectural equipment. Where stainless steel has become extremely dirty with signs of surface discolouration (perhaps following periods of neglect, or misuse) alternative methods of cleaning can be used, as outlined below.

Does stainless steel corrode?

Although stainless steel is much more resistant to corrosion than ordinary carbon or alloy steels, in some circumstances it can corrode. It is 'stain-less' not 'stain-impossible'. In normal atmospheric or water based environments, stainless steel will not corrode as demonstrated by domestic sink units, cutlery, saucepans and work-surfaces.

Is stainless steel non-magnetic?

This is not strictly true and the real situation is rather more complicated. The degree of magnetic response or magnetic permeability is derived from the microstructure of the steel. A totally non-magnetic material has a magnetic permeability of 1. Austenitic structures are totally non-magnetic and so a 100% austenitic stainless steel would have a permeability of 1.

How to choose which stainless steel to use?

Most decisions about which steel to use are based on a combination of the following factors:

A. What is the corrosive environment?

Atmospheric, water, concentration of particular chemicals, chloride content, presence of acid.

B. What is the temperature of operation?

High temperatures usually accelerate corrosion rates and therefore indicate a higher grade. Low temperatures will require a tough austenitic steel.

C. What strength is required?

Higher strength can be obtained from the austenitic, duplex, martensitic and PH steels. Other processes such as welding and forming often influence which of these is most suitable.

D. What welding will be carried out?

Austenitic steels are generally more weldable than the other types. Ferritic steels are weldable in thin sections. Duplex steels require more care than austenitic steels but are now regarded as fully weldable. Martensitic and PH grades are less weldable.

E. What degree of forming is required to make the component?

Austenitic steels are the most formable of all the types being able to undergo a high degree of deep drawing or stretch forming. Generally, ferritic steels are not as formable but can still be capable of producing quite intricate shapes. Duplex, martensitic and PH grades are not particularly formable.

F. What product form is required?

Sheet, bar, tube. In general, the austenitic steels are available in all product forms over a wide range of dimensions. Ferritics are more likely to be in sheet form than bar. For martensitic steels, the reverse is true.

G. What are the customer's expectations of the performance of the material?

This is an important consideration often missed in the selection process. Particularly, what are the aesthetic requirements as compared to the structural requirements? Design life is sometimes specified but is very difficult to guarantee.

H. Non-magnetic properties

There may also be special requirements such as non-magnetic properties to take into account.

I. It must also be borne in mind that steel type alone is not the only factor in material selection.

Surface finish is at least as important in many applications, particularly where there is a strong aesthetic component.

J. Availability.

There may be a perfectly correct technical choice of material which cannot be implemented because it is not available in the time required.

K. Cost.

Sometimes the correct technical option is not finally chosen on cost grounds alone. However, it is important to assess cost on the correct basis. Many stainless steel applications are shown to be advantageous on a life cycle cost basis rather than initial cost.

Information require when choose a filter housing

In order to choose filter housing correctly, the following information require. The more information we have, the more precise the housing and filter media we recommend.

What is the operating and maximum flow rate require?

Maximum flow rate 50m�/hr require. We will size a housing with flow rate average about 30% higher than 50m�/hr, a housing able to allow maximum flow rate 65m�/hr.

What is the filtering liquid?

Normally a 10"�, 1 micron filter cartridge's average flow rate is about 1m�/hr for cleaner and lower viscosity liquid.

What is the operating and maximum temperature of the liquid?

For ambient or below 50� C liquid, silicon seal can be use, but if the temperature is above 50� C, viton seal is recommended.

How viscosity is the liquid?

The higher viscosity, the lower flow rate allow for filter media.

What is the pH of the liquid?

For neutral or almost neutral liquid, housing with SUS 304 wetted part and silicon seal can be use. Strong pH liquid.

What is the operating pressure and maximum pressure require?

The higher pressure, the thicker housing body, flange and other parts require.

Liquid (media) analysis report available?

We can get data like pH, hardness, silica and other in the report. This will help to choose a right filtration media and filter housing.

What usage of the after filtered liquid?

For irrigation, 5 or 10 micron filter bag is good enough to remove particles which will clog the irrigation nozzles. In electronic industries, for wafer washing, the final filter housing may need in SUS 316L material and the filtration micron size may as low as 0.2 or 0.1 micron.

The filter housing to be use for which industries?

Final process of pharmaceutical or F & B industries may need SUS 316L material housing with sanitary connections. While SUS 304 is good enough for normal coarse filtration.

What is the filtration media (filter cartridge or filter bag) to be use?

We will propose according to your requirement, or we will recommend the right filter housing and filter media base on the above information.

What is the filtration micron size require?

We will propose according to your requirement, or will recommend the right micron size base on the above information.

What is the material of the piping which going to connect to the filter housing?

Either the above information require is complete or lack of information, this will help us to understand better and recommend the right filter housing and filter media.

Stainless Steel Designations

Water Hammer

Water hammer commonly occurs when a valve is closed suddenly at an end of a pipeline system, and a pressure wave propagates in the pipe.

The magnitude of the pulse

Water hammer can be analyzed by two different approaches, rigid column theory which ignores compressibility of the fluid and elasticity of the walls of the pipe, or by a full analysis including elasticity.

Effects and mitigation

If the pipe is suddenly closed at the outlet (downstream), the mass of water before the closure is still moving forward with some velocity, building up a high pressure and shock waves.

Dynamic Equations

The water hammer effect can be simulated by solving the following partial differential equations where V is the fluid velocity inside pipe, ρ is the fluid density and Bm is the equivalent bulk modulus, f is the friction factor.

Possible causes

Sudden valve closure, Pump failure , Check valve slam (due to sudden deceleration, a check valve may slam shut rapidly, depending on the dynamic characteristic of the check valve and the mass of the water between a check valve and tank).

Mitigating measures

Water hammer has caused accidents and fatalities, but is usually less threatening. In many cases damage is limited to breakage of pipes or appendages.

Software

Software packages vary in complexity, dependent on the processes modeled. The more sophisticated packages may have any of the following features:

What forms of corrosion can occur in stainless steels?

The most common forms of corrosion in stainless steel are:

A. Pitting corrosion

The passive layer on stainless steel can be attacked by certain chemical species. The chloride ion Cl- is the most common of these and is found in everyday materials such as salt and bleach. Pitting corrosion is avoided by making sure that stainless steel does not come into prolonged contact with harmful chemicals or by choosing a grade of steel which is more resistant to attack. The pitting corrosion resistance can be assessed using the Pitting Resistance Equivalent Number calculated from the alloy content.

B. Crevice corrosion

Stainless steel requires a supply of oxygen to make sure that the passive layer can form on the surface. In very tight crevices, it is not always possible for the oxygen to gain access to the stainless steel surface thereby causing it to be vulnerable to attack. Crevice corrosion is avoided by sealing crevices with a flexible sealant or by using a more corrosion resistant grade.

C. General corrosion

Normally, stainless steel does not corrode uniformly as do ordinary carbon and alloy steels. However, with some chemicals, notably acids, the passive layer may be attacked uniformly depending on concentration and temperature and the metal loss is distributed over the entire surface of the steel. Hydrochloric acid and sulfuric acid at some concentrations are particular aggressive towards stainless steel.

D. Stress corrosion cracking (SCC)

This is a relatively rare form of corrosion which requires a very specific combination of tensile stress, temperature and corrosive species, often the chloride ion, for it to occur. Typical applications where SCC can occur are hot water tanks and swimming pools. Another form known as sulphide stress corrosion cracking (SSCC) is associated with hydrogen sulphide in oil and gas exploration and production.

E. Inter granular corrosion

This is now quite a rare form of corrosion. If the Carbon level in the steel is too high, Chromium can combine with Carbon to form Chromium Carbide. This occurs at temperatures between about 450-850 deg C. This process is also called sensitization and typically occurs during welding. The Chromium available to form the passive layer is effectively reduced and corrosion can occur. It is avoided by choosing a low carbon grade the so-called 'L' grades or by using a steel with Titanium or Niobium which preferentially combines with Carbon.

F. Galvanic corrosion

If two dissimilar metals are in contact with each other and with an electrolyte e.g. water or other solution, it is possible for a galvanic cell to be set up. This is rather like a battery and can accelerate corrosion of the less 'noble' metal. It can avoided by separating the metals with a non-metallic insulator.