Monday, December 1, 2008

Material Construction

There are many kind of material that used at Heat Exchanger fabrication. Such as Carbon Steel,Austenite steel, Duplex Stainless steel ,Super Austenite and Titanium. Let we learn about this material more clearly.
1. Carbon Steel
Carbon steel, also called plain carbon steel, is steel where the main alloying constituent is carbon. The AISI defines carbon steel as: "Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, columbium [niobium], molybdenum, nickel, titanium, tungsten, vanadium or zirconium, or any other element to be added to obtain a desired alloying effect; when the specified minimum for copper does not exceed 0.40 per cent; or when the maximum content specified for any of the following elements does not exceed the percentages noted: manganese 1.65, silicon 0.60, copper 0.60."[1]

The term "carbon steel" may also be used in reference to steel which is not stainless steel; in this use carbon steel may include alloy steels.
Steel which is Fe plus a small amount of C has crystal structure at room temperature called Ferrite. Ferrite has one atom Fe atom at each corner of a cube and another in the centre of the body and this is called Body Center Cubic.
Types of carbon steel

Typical compositions of carbon are:

* Mild (low carbon) steel: approximately 0.05% to 0.26% carbon content with up to 0.4% manganese content[1] (e.g. AISI 1018 steel). Less strong but cheap and easy to shape; surface hardness can be increased through carburizing.[2]

* Medium carbon steel: approximately 0.29% to 0.54% carbon content with 0.60 to 1.65% manganese content[1](e.g. AISI 1040 steel). Balances ductility and strength and has good wear resistance; used for large parts, forging and automotive components.[3]

* High carbon steel: approximiately 0.55% to 0.95% carbon content with 0.30 to 0.90% manganese content..[1] Very strong, used for springs and high-strength wires.[4]

* Very high carbon steel: approximately 0.96% to 2.1% carbon content, specially processed to produce specific atomic and molecular microstructures.[1]

Steel can be heat-treated which allows parts to be fabricated in an easily-formable soft state. If enough carbon is present, the alloy can be hardened to increase strength, wear, and impact resistance. Steels are often wrought by cold-working methods, which is the shaping of metal through deformation at a low equilibrium or metastable temperature.

2. Stainless steel
when CR is added to steel to make stainless steel, the ferritic crystal structure doesn't change since Cr is a ferrite former. It wishes to form the ferritic crystal structure. These Fe-Cr are called the ferritic stainless steel.
In metallurgy, stainless steel is defined as a steel alloy with a minimum of 10% chromium content by mass.[1] Stainless steel does not stain, corrode, or rust as easily as ordinary steel (it "stains less"), but it is not stain-proof. It is also called corrosion-resistant steel when the alloy type and grade are not detailed, particularly in the aviation industry. There are different grades and surface finishes of stainless steel to suit the environment to which the material will be subjected in its lifetime. Common uses of stainless steel are cutlery and watch straps.

Stainless steel differs from carbon steel by amount of chromium present. Carbon steel rusts when exposed to air and moisture. This iron oxide film is active and accelerates corrosion by forming more iron oxide. Stainless steels have sufficient amount of chromium present so that a passive film of chromium oxide forms which prevents further corrosion.
Types of stainless steel

There are different types of stainless steels: when nickel is added, for instance, the austenite structure of iron is stabilized. This crystal structure makes such steels non-magnetic and less brittle at low temperatures. For greater hardness and strength, carbon is added. When subjected to adequate heat treatment, these steels are used as razor blades, cutlery, tools, etc.

Significant quantities of manganese have been used in many stainless steel compositions. Manganese preserves an austenitic structure in the steel as does nickel, but at a lower cost.

Stainless steels are also classified by their crystalline structure:

* Austenitic, or 300 series, when Ni is added to Stainless steel, major changes occur since Ni is an "austenite former". The stainless steel wishes to form a different crystal structure call austenite. stainless steels comprise over 70% of total stainless steel production. They contain a maximum of 0.15% carbon, a minimum of 16% chromium and sufficient nickel and/or manganese to retain an austenitic structure at all temperatures from the cryogenic region to the melting point of the alloy. A typical composition of 18% chromium and 10% nickel, commonly known as 18/10 stainless, is often used in flatware. Similarly, 18/0 and 18/8 are also available. Superaustenitic stainless steels, such as alloy AL-6XN and 254SMO, exhibit great resistance to chloride pitting and crevice corrosion due to high molybdenum content (>6%) and nitrogen additions, and the higher nickel content ensures better resistance to stress-corrosion cracking versus the 300 series. The higher alloy content of superaustenitic steels makes them more expensive. Other steels can offer similar performance at lower cost and are preferred in certain applications.[citation needed]

The low carbon version of the Austenitic Stainless Steel, for example 316L or 304L, are used to avoid corrosion problem caused by welding.The vast majority of stainless steels used in the world are this type. The "L" means that the carbon content of the Stainless Steel is below 0.03%, this will reduce the sensitization effect, precipitation of Chromium Carbides, due to the high temperature produced by welding operation.
The austenite crystal structure is important because it is formable, welavle and tough.

* Ferritic stainless steels are highly corrosion-resistant, but less durable than austenitic grades. They contain between 10.5% and 27% chromium and very little nickel, if any, but some types can contain lead. Most compositions include molybdenum; some, aluminium or titanium. Common ferritic grades include 18Cr-2Mo, 26Cr-1Mo, 29Cr-4Mo, and 29Cr-4Mo-2Ni.

* Martensitic stainless steels are not as corrosion-resistant as the other two classes but are extremely strong and tough, as well as highly machineable, and can be hardened by heat treatment. Martensitic stainless steel contains chromium (12-14%), molybdenum (0.2-1%), nickel (0-<2%), and carbon (about 0.1-1%) (giving it more hardness but making the material a bit more brittle). It is quenched and magnetic.

* Precipitation-hardening martensitic stainless steels have corrosion resistance comparable to austenitic varieties, but can be precipitation hardened to even higher strengths than the other martensitic grades. The most common, 17-4PH, uses about 17% chromium and 4% nickel. There is a rising trend in defense budgets to opt for an ultra-high-strength stainless steel when possible in new projects, as it is estimated that 2% of the US GDP is spent dealing with corrosion. The Lockheed-Martin Joint Strike Fighter is the first aircraft to use a precipitation-hardenable stainless steel—Carpenter Custom 465—in its airframe.

* Duplex stainless steels have a mixed microstructure of austenite and ferrite, the aim being to produce a 50/50 mix, although in commercial alloys, the mix may be 40/60 respectively. Duplex stainless steel are created by adding only about half the amount of nickel which would normally be added to make a fully austenitic grade. Duplex steels have improved strength over austenitic stainless steels and also improved resistance to localised corrosion, particularly pitting, crevice corrosion and stress corrosion cracking. They are characterised by high chromium (19–28%) and molybdenum (up to 5%) and lower nickel contents than austenitic stainless steels. The most used Duplex Stainless Steel are the 2205 (22% Chromium, 5% Nickel) and 2507 (25% Chromium, 7% Nickel) sometimes the 2507 is also called "SuperDuplex" due to the higher Corrosion resistance.
Why Duplex Stainless Steel??
Keep the positives and reduce the negative
1. 300 series
a. General corrosion resistance
b. Ease of forming and welding
2. 400 series
a. Resistance to SCC (Stress Corrosion Cracking)
b. High strength

Material Option
1. Ferritic
a. Not available in all product forms
b. Difficult to fabricate
c. 400 series stainless
2. Higher Nickel Alloys
a. Easier to fabricate
b. Good availability
c. Can be expensive
d. Alloy 20, AL-6XN. 825, 625, C-276
3. Duplex
a. Relatively inexpensive
b. Easier to fabricate thhan 400 series
c. LDX 2101, 2205, 2507

All of above material have maximum temperature that can be handle. Maximum temperature limits by ASME Section VIII, Div 1 are :

316L Limitation :
Highly susceptible to chloride stress and corrosion cracking.




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