I'll take a shot.
This is really an interesting line of thinking though.
I'm not sure 'ties up' is a good language to use.
If memory serves: And this is perhaps just a little Gedanken here. OK, a whole lot.
It is true that in an indivdual matrix cell of martensite there is more carbon than in an individual matrix cell of ferrite. And maybe C don't move as easily in room temp martensite as it does in ferrite. I don't know.
But if we are talking about long term relatively moderate temp evolutions I think the martensitee wants to be something else. Is more unstable than ferrite. So just as a guess, even though it may theoretically 'tie up' more carbon initially, I think it would want to give it up easier than ferrite.
OK, thats my pathetic attempt for the day. This type of stuff is way over the heads of anybody that frequents this room. And I believe that even though the exercise of resolving this would be a very fascinating pursuit, the utlimate conclusion will be so metallurgically complicated that 10 metallurgists would probably come up with 20 different opinions.

PART III

Okay so let's start out with Martensitic class of stainless steels:

The martensitic class of stainless steels depend primarily on Cr from 11.5% to 18% as the major alloying element. This class and the ferritic stainless steel class share the AISI 400 numbering series. They are sometimes referred to as straight chromium grades. In contrast, Austenitic stainless steels are essentially chromium-nickel alloys; they are covered by the AISI 300 series for most standard compositions of wrought products. AISI 410, the principal alloy of the martensitic class, ranges 11.5 to 13.5% Cr. However, casting grade CA-6NM, provides higher strength, increased toughness, better weldability and greater corrosion resistance than the CA-15 (Type 410) casting grade.

Martensitic stainless steels exhibit magnetic properties similar to those of plain carbon steels and therefore, are also subject to arc blow in welding. Unlike mild steels, they are air hardening when allowed to cool rapidly from the austenitizing temperature range (1600-1850 F) where the austenite phase is predominant. Austenitizing temperature are reached in the heat affected zones of weldments and subsequent cooling occurs at rates sufficient to produce martensite. In the annealed condition martensitic stainless steels have basically a ferritic microstructure with dispersed chromium carbides.

Carbon and chromium in the martensitic stainless steels act in concert to prevent transformation of austenite to ferrite during rapid cooling. The result is the distorted BCT structure called martensite which is like a BCC but elongated in one direction. The heat affected zone on either side of a weldment will develop the hard, brittle martensite phase and, unless local preheating is practiced, it may develop cracks due to shrinkage stresses and hydrogen. The hardness of the HAZ depends primarily on the carbon content of the base metal. Increased hardness results in decreased toughness and more susceptibility to cracking. Although the most suitable preheat and interpass temperature will depend on carbon content, the mass of the joint, degree of restraint and the filler metal composition... Temperatures of 400 -600 F are commonly specified.

Compared with plain carbon steels, martensitic stainless steels have higher electrical resistance, lower thermal conductivity and, when rapidly cooled, a brittle structure. Preheating of the base/parent metal retards the rate of cooling, permitting the weld metal and Heat Affected Zones (HAZ) to cool at a slower and more uniform rate, thereby reducing shrinkage stresses. The slower cooling also allows more of the hydrogen to escape. Postweld heating at 1300-1400 degrees Fahrenheit, followed by controlled cooling at a rate of 50 degrees F per hour to 1100 F before air cooling, is also desirable to temper the martensitic structure in the weldment (reducing hardness and increasing ductility and corrosion resistance). Where possible, postweld heating should be done before the weldment cools down.

For optimal results, the weldment should not be allowed to cool down below the preheating temperature between passes or prior to postweld heating. Where 410NiMo filler metals (Somewhat less hardenable than 410) or austenitic filler metals such as 309 or 312 are used, preheating and postheating procedures may be less demanding than with 410 filler metals. In special circumstances austenitic grades are sometimes specified where the differences in composition and physical properties such as coefficient of of thermal expansion are acceptable for the application.

If preheating or postweld heating cannot be done or are impractical , austenitic filler metals such as 309 or 312 can be specified to give somewhat more assurance that cracking will be averted. The AISI 500 series (e.g. 502 with 5% Cr, .5% Mo and 505 with 9% Cr, 1% Mo) heat resisting steels, although not classed as stainless due to Cr being under 10-11% minimum, are nevertheless martensitic and require essentially the same tender loving care (TLC) as martensitic stainless steels like 410.

Martensitic steels, being lower in alloy content, are lower in cost than austenitic stainless steels. When suitably heat treated, they have adequate corrosion resistance in many environments and also offer high strength and good fatigue properties together with excellent wear, oxidation and erosion resistance. They are adaptable for moderately high temperature service because of good tensile and creep strength at moderately elevated temperatures. Creep strength is the slow deformation of a metal for long periods of time at elevated temperatures under stresses which are less than the yield point. Typical applications include type 403 for turbine blades (high velocity fluid flow). Thpe 410 for valve seat facing and types 420 and 431 for cutlery grades, razor blades and surgical instruments although recently, newer and better alloy "recipes" have replaced the use of most of these grades in the most extreme applications.

Here is a very good and concise .pdf file by the Specialty Steel Industry of North America named the "Stainless Steel Information Handbook" so, here's the file to view, and if you want to download it, you need to go to the next link after this one in order to fill in a quick survey in order to download it for free:

This one is for viewing only because the copy and print features are disabled:

http://www.ssina.com/view_a_file/weldingbook.pdf

This one connects you to the page where you only need to fill out the quick survey and it will then start the download where all features are available:

http://www.ssina.com/publications/welding.html

There are plenty more links where a "boat load' of rather good information regarding all types of stainless steels can be found. Simply go to the search feature and type in stainless steel,  and you should be able to find threads where I have included many of these links in here for your review. I'll also post some newer links if I can find them later on. :) :) :) One site that's really good is this one which has many other links inside to give you as much detail as one desires:

http://www.msm.cam.ac.uk/phase-trans/2005/Stainless_steels/stainless.html

Here's one that may be of interest to OBEWAN in particular although everyone else is 'weldcome" to it as well:

http://www.msm.cam.ac.uk/phase-trans/2003/dominique.html

Finally for now ;), this one is an excellent resource with respect to welding metallurgy as it covers as well as lists many articles covering many different applications:

http://www.msm.cam.ac.uk/phase-trans/2002/welding.1.html

Now, let's continue to cover Ferritic Stainless Steels:

Note that the AISI 400 series covers the ferritic as well as the martensitic stainless steels. As Cr content is increased beyond the 11.5 to 18% range of the martensitic class the predominant metallurgical structure is ferrite even at elevated temperatures. In some grade there is enough C and nitrogen (N) to result in some austenite at high temperatures and hence partial hardening on rapid cooling. However generally speaking, the ferritic stainless steels are relatively non-hardening.

At a Cr level of 16 or 17% the structure at room temperature could essentially be ferritic with relatively low C (e.g. 430 with .12% C Max.) or essentially martensitic with relatively high C (e.g. 431 with .20% C Max.) Indeed, even with Cr as low as 10.55 and c less than .08% together with addition of about .50 - .75% Ti, the structure is still predominantly ferritic. some of the newer so-called super ferritic grades produced by AOD (Argon-Oxygen Decarburization) and vacuum melting techniques with quite low C & N contents (about .02% C and .02% N) offer out standing resistance to stress corrosion cracking in chloride solutions.

Ferritic stainless steels, like carbon steels and martensitic stainless steels, are quite magnetic and thus subject to arc blow in welding. In some of the standard ferritic grades residual carbon and nitrogen can combine with chromium to form carbiddes and nitrides at grain boundaries. This causes chromium depletion and in some cases, intergranular corrosion although not to the extent experienced with austenitic stainless steels. Ferritic stainless steel products are usually annealed at the mill to make sure that any martensite present may be transformed to the softer structure of ferrite and dispersed chromium carbides. However, subsequent welding may produce small amounts of austenite in the heat affected zones which on cooling could transform to martensite thus reducing ductility, toughness and corrosion resistance. The only redeeming feature of the martensite thus formed is that it tends to inhibit ferrite grain coarsening.

Grain coarsening is a fact of life with ferritic stainless steels. Heating above about 1700 F causes enlargement of ferrite grains with consequent embrittlement due to the loss of ductility and toughness. Since there is no phase change (Ferrite doesn't change into austenite) there is no chance of grain refinement. Embrittlement can also occur when ferritic stainless steels are held within a temperature range of 750-1050 F (most crucially at 885 F). The effect increases with increased Cr content. However, heating to about 1100 F for a short time followed by a rapid cooling through the 1050-750 F range will reverse the condition. A third contributor to embrittlement is sigma phase, an intermetallic compound of Fe& Cr which originates in the grain boundaries. Given enough time in the damaging temperature range of 1000 to 1700F, particularly around 1200 F, sigma phase can extend completely through entire grains

Chi phase, another embrittling intermetallic compound. cna occur along with sigma phase when molybdenum is present. From the foregoing it is obvious that prolonged heating of ferritic stainless steels within the 700 01700 F range should be avoided. Welding heat input should be minimized and slow cooling form welding avoided. Despite this, preheating of 300-450 F is recommended for welding when thickness exceeds about 1/4 of an inch, since ductility of the base metal is improved in that temperature range - particularly in the heat affected zone which may contain some martensite.

Postweld heating or PWHT of the low chromium ferritic stainless steels at 1450 to 1550 f will assure a wholly ferritic structure and partially restore mechanical properties and corrosion resistance that may have been adversely affected by the welding temperatures. However, exposure within this temperature range will quickly produce embrittling sigma phase in the higher chromium alloys such as 444 or 26-1. To minimize distortion, cooling may be done in the furnace down to no lower than 1100 f but prolonged exposure in the 1050 - 750 F range should be avoided due to 885 brittleness.

Because of embrittlement problems the ferritic stainless steels for the most part are not considered readily weldable and are used primarily for nonstructural applications. They are recommended for resistance to chloride stress corrosion cracking, corrosion in the aqueous media, oxidation at high temperature and pitting and crevice corrosion in chloride media. Applications include automobile exhaust equipment, radiator tanks, catalytic reactors, culverts, dry fertilizer tanks and animal containment housings. Type 430 is used for decorative trim, nitric acid tanks, and annealing baskets. Type 442 is used for components requiring protection from scaling at high temperatures such as furnace parts, nozzles, combustion chambers.

Austenitic filler metals such as 309, 310 and 312 are often used where the application can reconcile the different corrosion resistant characteristics and the greater coefficients of linear expansion of the austenitic grades. Where postweld annealing at 1450 F is specified, the austenitic filler metal should be either a stabilized or low carbon grade to avoid carbide perciptation. Tomorrow, I'm going to post about the Austenitic class of stainless steels so you all can soak all of this in because this class is a whole bunch different than any of the first two classes we have already covered which are the Martensitic and the Ferritic classes of stainless steels.

Respectfully,
Henry