[s-cars] Stainless-Exhaust have enough yet?
Jeff.Postupack at analog.com
Wed Jun 27 12:22:14 EDT 2007
I am not sure this ever made it to the List!
From: Michael Bess [mailto:mlbess at optonline.net]
You'll probably wished you never got me involved in this :-).
304 stainless (18% Chromium, 8% Nickel) is the standard first choice for corrosion resistance in many applications. I was not up on 439 so I need to do some research (citations below). A lot of OEM auto manufactures started using 409 stainless (11% Chromium) a number of years ago for improved corrosion resistance over plain carbon steel. Then they started to use aluminized 409 (think of hot dip galvanizing galvanizing but using aluminum) to get more corrosion resistance. Then there was the development of 439 (18% Chromium, 0.4% Titanium) as an improvement over 409 with better corrosion resistance.
Costs have indeed skyrocketed as you have noted, both due to Chinese appetite for materials as well as other international demand and supply side constraints (be thankful you do not need Titanium or aerospace quality maraging steels).
439 is actually a good choice for the pipes. It expands less than 304, dissipates heat faster, is readily welded and has good high temperature oxidation resistance. Thus its use for both pipes and the cat and resonator is ideal.
These systems should last as long as mechanical issues are not a problem. From a corrosion perspective (both high temperature and road salt related) 439 will work just fine. We could always consider a Titanium exhaust system like used on the Corvette (even lighter weight).
My metallurgical opinion.....
METALS MARKETING & TECHNOLOGY
A good pdf brochure which shows some corrosion data for 409, 439 and 304 stainless.
Comparative study of high temperature oxidation behaviour in AISI 304 and AISI 439 stainless steels
This work deals with a comparison of high temperature oxidation behaviour in AISI 304 austenitic and AISI 439 ferritic stainless steels. The oxidation experiments were performed between 850 and 950 °C, in oxygen and Ar (100 vpm H2). In most cases, it was formed a Cr2O3 protective scale, whose growth kinetics follows a parabolic law. The exception was for the the AISI 304 steel, at 950 °C, in oxygen atmosphere, which forms an iron oxide external layer. The oxidation resistance of the AISI 439 does not depend on the atmosphere. The AISI 304 has the same oxidation resistance in both atmospheres, at 850 °C, but at higher temperatures, its oxidation rate strongly increases in oxygen atmosphere. Concerning the performance of these steels under oxidation, our results show that the AISI 439 steel has higher oxidation resistance in oxidizing atmosphere, above 850 °C, while, in low pO2 atmosphere, the AISI 304 steel has higher oxidation resistance than the AISI 439, in all the temperature range investigated.
This grade of stainless steel can be used where the tube length is less than three metres. The coefficient of thermal expansion for 304 is 1.8×10-2 mm/m/°C which is substantial more than that of carbon steel. When the vessel is hot the thermal stresses in the tubes will be high. Tubes of 304 stainless steel should always be annealed after welding.
439 Stainless Steel
ASTM TP439 is a titanium stabilised ferritic grade of stainless steel (17-19% Cr) which is recommended for long evaporator or pan tubes (in excess of 5m long)
Advantages of grade 439
* fully ferritic metallurgical structure (ensured by the titanium stabilisation)
* very good weldability and ductility;
* inter-crystalline corrosion resistance;
* pitting corrosion resistance;
* full immunity to stress corrosion.
* Coefficient of thermal expansion (in the range 0°C - 100°C) is 1.02×10-2 mm/m/°C
Stress corrosion cracking
This type of corrosion occurs when
* A susceptible material is subject to
* mechanical stress in a
* corrosive environment
In an evaporator, pan or juice heater under the above conditions the result will be cracks leading to breakage in the area near the tube plate.
The danger of stress corrosion cracking exists in virtually all evaporators. The risk will be higher if tubes over 7 metres in length (some designs of continuous pans, falling-film evaporators and Kestner evaporators). Ferritic stainless steels are immune to stress corrosion cracking
Thermal conductivity of ferritic stainless material is 40% higher than that of austenitic grades (like 304, 304L, 316 or 316L) i.e.: 26 vs 15 watt/metre/°C.
From Allegheny Ludlum:
Subcategory: T 400 Series Stainless Steel; Stainless Steel; Metal
Key Words: Pipe & Tubing: ASTM A268, AMSME SA268, ASTM A803. Flat Rolled: ASTM A240; ASME SA240. Bar: ASTM A479, ASME SA479
Iron content above calculated as balance.
Allegheny Ludlum Stainless Steel Type 439, a titanium stabilized, 18 percent chromium alloy, also known as ASTM XM-8 and by the UNS designation S43035, is a ferritic stainless steel designed to resist corrosion in a variety of oxidizing environments from fresh water to boiling acids. It may be used in either the annealed, cold formed or as-welded condition. Type 439 may also be used in many oxidizing environments where Type 304 is considered adequate in term of general corrosion resistance but is subject to chloride stress corrosion cracking.<\p>
The composition of Type 439 stainless steel has been balanced to provide a completely ferritic structure at all temperatures, to avoid the loss of ductility after welding and to provide resistance to intergranular corrosion. Type 439 does not require annealing after welding to restore ductility or to provide intergranular corrosion resistance.
Excellent resistance to stress corrosion cracking, good weldability, high thermal conductivity and low thermal expansion characteristics make Type 439 stainless steel and ideal consideration for many applications. For shell and tube heat exchangers where carbon steel shells are used with Type 439 tubes the close match of thermal expansion coefficients may possibly eliminate the need for an expansion joint in the heat exchanger. Type 439 has a low carbon content making it an attractive consideration for nuclear applications. Type 439 also provides good oxidation and corrosion resistance for many automotive exhaust system components and residential furnace primary heat exchangers.
Information provided by Allegheny Ludlum Corporation.
More information about the S-CAR-List