So What is Ferrite in Stainless Anyway?

By ssbn727 (*****)

Date 11-21-2009 01:00

From a practical standpoint, Ferrite can be considered pure iron at a low temperature.

Sigma phase is a nonmagnetic intermetallic phase composed mainly of Iron and chromium which forms in ferritic and austenitic stainless steels during exposure at temperatures of 1050 to 1800 degrees F (560 to 980 C ) It causes loss of ductility, toughness and is generally strain intolerant, or brittle at temperatures under 250 to 300 degrees F ( 120 - 150 C ) but it is believed it has little effect on properties in the temperature range where it forms. If this is so, it would appear to have little consequence as long as the affected components continuously operate at the elevated temperature. However cracking could occur if the components were impact loaded, or excessively stressed during maintenance work.

Respectfully,
Henry

By MMyers

Date 11-22-2009 00:15

Here's some verbiage from an old Stoody book titled Stainless Welding Engineering (1977):
"For the Iron-Carbon system:
There are two ferrite regions separated by an austenite area. The upper area is delta ferrite, the lower alpha ferrite. Ferrite is a solid solution of carbon in B.C.C. iron."

And from Welding Metallurgy of Stainless and Heat Resisting Steels, English translation in 1975:
"Iron - Chromium system:
(referring to the Fe-Cr phase diagram) A portion situated outside a second loop enveloping the first one, where the body centered cubic ferrite phase ('alpha' or 'delta' phase) is stable from room temperature up to the melting point of the alloy. By analogy, with iron and ordinary steels, ferrite formed on solidification is known as 'delta ferrite', while ferrite resulting from transformation of austenite during cooling is termed 'alpha ferrite'."

So, based on these (and the appropriate phase diagram), I would say (guess) that in your common stainless steel, you get a mesh of these two answers: ferrite, a BCC crystal primarily consisting of iron with additions of carbon, chromium, and other elements based on the chemical composition of the alloy and partitioning of elements. So like all metallurgical answers, it depends.

By ssbn727 (*****)

Date 11-22-2009 08:17 Edited 11-23-2009 14:01

Hi Mike!

Okay I did some checking because I do agree that there is a very slight amount of carbon in Ferrite, so I went to my library in my house and opened this book titled: "Elementary Metallurgy and Metallography" Third edition, by Arthur M. Shrager, published by Dover, third edition, 1969, ISBN # 0-486-60138-2, and it states this on page 37: " A solid solution of which alpha iron is the solvent is called Ferrite. Because of the extremely small amount of carbon which it can contain in solid solution, ferrite in a steel that contains only iron and carbon may be considered substantially pure iron, and it is called pure iron in some texts.

Because the ferrite of an alloy steel may contain in solid solution appreciable amounts of other elements, however, it is better to use the term only in it's exact meaning: a solis solution of any element in alpha iron. Then I looked in the back of the book in the Glossary, and the definition states basically the same except to add that the amount of carbon in solid solution is to be less than five hundredths of a percent and minor amounts of other elements as well, yet like what was written on page 37: " In it's exact meaning, a solid solution of any element in Alpha Iron." So this means that Alpha Iron is really pure iron as an element at room temperature... What is a solid solution you ask? It is an alloy in which metals remain dissolved in each other when solid

And so I decided to look at an even older metallurgy book I just happen to have also which is: "Metallurgy" By Carl G. Johnson Associate Professor of mechanical Engineering Worcester Polytechnic Institute, Consultant and Member of the American Society of Metals and American Welding society, Fourth Edition, published in 1956 by The American Technical Society... So I also look in the glossary for ferrite, and what is written in this book is for "Free Ferrite" which is defined as: Iron phase found in a steel which has less than eighty five hundredths of a percent of carbon that has been slow cooled from above it's critical temperature range; the iron phase is alpha iron, low in carbon content.

So it really does depend which book you're reading from with respect to ferrite ,and of course there's also delta ferrite, alpha ferrite as well as acicular ferrite too which can lead to confusion!!! :) :) :) Since I'm experiencing an unusally intense amount of pain right as I'm typing this now which feels like I'm being stabbed all over the place as well as my skin feeling like it's on fire. I will stop here and encourage you to purchase a comprehensive fundamental metallurgy book which you can read through at your leisure! ;)

Now Cementite is a chemical compound of iron & carbon, also known as iron carbide (Fe3C)which contains about 6.8 percent carbon. It occurs as grain envelopes or as needles within a grain of hypereutectoid steel. It occurs as lamellae in pearlite. It may also occur as spheroids in annealed steel. It is extremely hard and brittle. So, it should not be associated or confused with being the same as ferrite. I would go on to further elaborate more on Sigma phase but, the pain is getting too intense now, so I'll stop here!!! Maybe tomorrow I'll feel better enough to continue... We'll see. ;)

Respectfully,
Henry

By ssbn727 (*****)

Date 11-22-2009 09:31

I took so me pain meds and they're starting to work a little bit so, I decided to include this inot htis thread from an earlier set of posts I made earlier in the month regarding the different grades of stainless steels, and I though that this may be beneficial in helping you understand the two terms you inquired about, so here goes:

From the the Mckay book on welding Stainless steels... Part one

"A GRAIN OF TRUTH... WHENCE COMETH GRAINS AND GRAIN BOUNDARIES???"

Before discussing the major classes of stainless steels and appropriate welding filler metals it would be prudent to review the mechanism of solidification and subsequent formation of grains and grain boundaries. When a liquid steel cools through the solidification temperature range peculiar to that particular grade of steel, the process of crystallization commences. Although individual atoms have great freedom (mobility) in the liquid state, once freezing starts they begin to orient themselves in a specific type of crystal lattice or arrangement.

Nucleation of crystals occurs simultaneously in various locations. The pattern in each location repeats itself. Crystal growth resemble pine tree formations, and is described as dendritic branching or dendrites. During the process of solidification the separate crystals, growing at various rates in different locations (each in their own orientation or plane of atoms) converge to form boundaries. The individual areas of uniform crystal structure are called grains and the boundary areas, grain boundaries. Grain size and shape are determined by the manner in which the branches from dendrites meet.

Grain boundaries represent junctions of differently oriented crystals or regions of disarray in the atomic arrangement. The atomic structure in converging grains at random orientation cannot match perfectly at their interfaces. Therefore between any grains there exists a transition layer where the atoms are not in their proper places with respect to either grain . Exposure to a chemical etching solution results in preferential attack in these vulnerable grain boundary locations. Thus the the metallurgist can prepare and etch a metallographic specimen from stainless steel base/parent metal or a weld deposit, his or her crystal ball for determining the intricacies of grain size, shape, structure, etc., is called a microscope and more recently, a Scanning Electron Microscope or SEM.

Subsequent to solidification, say after cooling to room temperature, grain growth - where larger grains grow at the expense of smaller ones - may occur during heating and cooling cycles. Explanation of this phenomenon lies in the realm of thermodynamics involving so called free energy at grain interfaces. Above the recrystallization temperature, grain growth occurs because grains have a tendency to decrease their surface energy. Large grains have smaller grain boudary area per unit volume hence smaller surface energy. Because larger grains grow at the expense of smaller ones, energy is conserved.

FERRITE, AUSTENITE, MARTENSITE & SOLID SOLUTIONS ... WHAT ARE THEY???

Since the three main classes of stainless steels are maned in accordance with their predominant metallurgical phases, i.e. Ferritic, Austenitic and Martensitic, it seems advisable to explain these phases before discussion each stainless steel class in further detail. an elementary knowledge of solid solutions in metals will also lead to a better understanding of stainless steels. So, we'll start with Ferrite and Austenite...

Pure Iron freezes at 2802 degrees F in a crystal form called ferrite. It has a body centered cubic structure (BCC) with the basic cell containing one atom at each of eight corners and one atom in the middle. On cooling to 2552 F, the ferrite structure transforms into a modification called austenite, the unit cell of which is face centered cubic (FCC) containing one atom at each of eight corners and one atom on each face (none in the middle). At 1670 F, the austenite phase reverts to ferrite, the form stable at room temperature.

Although structurally identical, the ferrite forming at the higher temperature is designated delta ferrite while the ferrite forming at a lower temperature is called alpha ferrite. Austenite is often designated as the gamma phase. So BCC Delta ferrite will start off with nine atoms at 2802 F and then will turn to Austenite FCC upon cooling to 2552 F made up of 14 atoms, and finally upon even more cooling down to 1670 F revert back to 9 atoms and once again turn back into a BCC structure - only this time it is designated as alpha ferrite instead.

Okay so, we'll start with MARTENSITE FIRST:

In commercial steels, the addition of carbon and alloying elements such as chromium will modify the structure at room temperature in accordance with composition and thermal history. Plain carbon steels, even when cooled rapidly from the austenitizing temperature, develop ferrite and pearlite. Under the microscope at high magnification, pearlite can be identified as alternate platelets of ferrite and iron carbide. The latter is sometimes described as Fe3C or Cementite. The FCC structure of austenite is more compact than the BCC structure of ferrite. During the Austenite to ferrite transformation an expansion occurs. Shifting of the atomic layers and groups cna cause "Foreign" atoms to trap and jam, leading to great distortion. when alloy steels containing sufficient carbon and alloy content are cooled rapidly enough from the austenitizing temperature, the distorted structure - called martensite is formed, producing high strength and hardness.

Martensite can be described as a distorted arrangement of attoms having a body centered teragonal (BCT) unit structure. During fast cooling, the normal transformation from austenite to ferrite that would be expected in a low carbon steel is hindered by the presence of higher carbon and alloying elements and the lack of time to reach a condition of greater equilibrium. Since martensitic structures as-formed are hard and brittle, subsequent heat treatment (Tempering) is necessary to reduce their hardness and strength and increase their ductility to he level required for most industrial applications.

SOLID SOLUTIONS:

We're all familiar with liquid solutions such as salt in water. If you add salt (the solute) to a cup of hot water (The solvent) the salt dissolves and forms a liquid solution. But if you then put the cup in the refrigerator and examine it several hours later you see a precipitate in the bottom, i.e. some of the salt has come out of the solution. You can say that the solubility of salt in water decreases with decreasing temperature....

The same type of thing occurs in steels. We call them solid solutions. For example, an alloy of iron with about 13% chromium consists of a random mixture of iron (Solvent) and chromium (Solute) atoms. Iron (Fe) and chromium (Cr) atoms are approximately the same size and about one in seven atoms will be Cr. This distribution of atoms in a crystal (grain) in which the atoms of the second element are evenly distributed in the parent crystal structure is known as a solid solution. Where the Cr atoms are to be found in exactly similar sites to the Fe atoms, the solid solution is known as substitutional.

When you add carbon (C) to the alloy, the C atoms - being smaller than the Fe or Cr atoms - distribute themselves in the gaps, or interstices between the other atoms, assuming C is under .1%. The C then has formed an interstitial solid solution. With c above about .1%, the C atoms in excess of the solubility limit may combine with Cr atoms to form Chromium Carbides.

Certain Interstices within the FCC structure of austenite are considerably more accommodating to C than those of the BCC structure of ferrite. Since the gaps between the atoms are larger in austenite than in ferrite, the solubility of C is greater in austenite but decreases abruptly when, with decreasing temperature, the austenite phase changes to ferrite or martensite. "Steels" are alloys of iron and carbon with modest quantities of manganese and silicon plus residuals - some which may be undesirable. Alloying elements such as chromium, nickle, molybdenum, etc., may be added for specific properties. Austenite in stainless steels is considered to be a solid solution of one or more elements in a FCC iron.

So now that we understand this better, lets look at briefly the standard classes of stainless steels...

Stainless steels are capable of meeting a wide range of final needs such as, corrosion resistance, strength at elevated temperatures, toughness at cryogenic temperatures, etc., and are selected for a broad range of applications. Designer need to be familiar with properties of various types available and should consider the advice of corrosion and welding engineers when specifying stainless steel grades for commercial service.

The three major classes of stainless steels that have been widely used in industrial application for over 75 (it says 50 in the booklet but I'm going to make it more current) are designated Martensitic (e.g. AISI 410), Ferritic (e.g. AISI 430) and Austenitic (e.g. AISI 304) - names derived from the predominant crystal structure or phase normally found at room temperature. A fourth class, the PH or Precipitation Hardening types (e.g. 17/4 PH)., has also come to prominence within the last 50 years or so. More recently, the Duplex, or Ferritic-Austenitic class (e.g. 2205 alloy). The next part after this is in another post

By ssbn727 (*****)

Date 11-22-2009 09:33

Here's part two:

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 abskets. 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 geater 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 diferent than any of the first two classes we have already covered whic are the Martensitic and the Ferritic classes of stainless steels.

Respectfully,
Henry

By ssbn727 (*****)

Date 11-22-2009 09:57 Edited 11-22-2009 18:12

I almost forgot! Here's the link to the original thread where I posted the last information on this thread:

http://www.aws.org/cgi-bin/mwf/topic_show.pl?pid=168082;msg=PstChange

I will be adding a new part IV covering the Austenitic Grades of Stainless Steels real soon. ;)

Respectfully,
Henry

By MMyers

Date 11-23-2009 21:03

"The same type of thing occurs in steels. We call them solid solutions. For example, an alloy of iron with about 13% chromium consists of a random mixture of iron (Solvent) and chromium (Solute) atoms. Iron (Fe) and chromium (Cr) atoms are approximately the same size and about one in seven atoms will be Cr. This distribution of atoms in a crystal (grain) in which the atoms of the second element are evenly distributed in the parent crystal structure is known as a solid solution. Where the Cr atoms are to be found in exactly similar sites to the Fe atoms, the solid solution is known as substitutional.

When you add carbon (C) to the alloy, the C atoms - being smaller than the Fe or Cr atoms - distribute themselves in the gaps, or interstices between the other atoms, assuming C is under .1%. The C then has formed an interstitial solid solution. With c above about .1%, the C atoms in excess of the solubility limit may combine with Cr atoms to form Chromium Carbides."

Very nice. The above is pretty much identical to what I had in my head when I wrote my post. I didn't think of this until this weekend, but the phase diagrams pretty much spell out all of the concentrations and such for solid solubility.

By tom cooper (**)

Date 12-07-2009 20:23

Hi Henry-

I just recently got back from a travel assignment and am only now catching up on things and wanted to thank you very, very much for the time you have put into answering my question.

After going through it all a couple of times I guess to sumarize what room temperature ferrite "is", is basically as you stated in your first post, i.e. regions of pure iron, except maybe very small % of C exists sometimes. Too much ferrite (FN > 10 in austenitics) adversely affects corrosion resisting) and too little (FN< ~2 in austenitics) can increase likely hood of hot cracking.

The next natural question is why/how does a small amount of ferrite (FN ~5) assist in prevention of cracking?

Thanks again.

By js55

Date 12-07-2009 22:07

Tom,
I'll take shot and see if my memory serves.
Ferrite doesn't. Delta ferrite does. Delta ferrite is the form of ferrite that forms upon solidification in those austenitic alloys with high enough Cr/Ni ratio for it to form. Generally around 1.8 to 2.0 or so, if memory serves. The low melt elements S, P, will solidify in the grain interiors of Delta ferrite and then migrate to the grain boundaries of the transforming austenite (some will have migrated to the delta ferrite grain boundaries) creating low melt strained grain boundaries upon further cooling.
The delta ferrite is the original solidfication ferrite and remains untransformed.
And it is actually even worse for those areas where the former ferrite grain boundaries have become austenite grain boundaries. If the delta ferrite does not transform then much of the low melt contituents remain in the grain interiors. And if memory serves as well the S/P fits in the delta ferrite better than in the austentie and will migrate slower.
Given that the austenite grains are considerably smaller than the delta ferrite grains this means that there is far more grain boundaries and shorter distances away for the S/P to migrate to.
OK.This shore ain't the gospel according to any authority just a shot at remembering some reading from many moons ago.

By MMyers

Date 12-07-2009 23:15

"The next natural question is why/how does a small amount of ferrite (FN ~5) assist in prevention of cracking? "

The explanation I have always been given (sitting in Lippold's class) is that P and S will help suppress the melting temperature of the final liquid to solidify (which is at the grain boundaries), and that liquid film will wet austenite/austenite grains better than it will wet austenite/ferrite grain boundaries. The application of stress by the solidifying weld causes and the inability for the liquid film to support tensile load causes a solidification crack. The reason you don't have this problem near as much with other austenitic materials (nickel based alloys) is that the P and S concentrations are much lower.

By ssbn727 (*****)

Date 12-08-2009 01:51

Hi Tom!

Both Jeff and Mike got it right, so I'm just going to expand a little on Ferrite...

FERRITE IN AUSTENITIC STAINLESS STEEL WELD METAL

What's so formidable about ferrite? It's not so formidable, It just makes things "tough." The structure of austenitic stainless steel weld metals varies from fully austenitic as in 310 to dual phase (Although NOT comparable to Duplex SS) austenitic-ferritic as in 308, 309, 312, 316 etc. which contain a small volume of the ferrite phase. In most cases some ferrite is desired for crack resistance. The extent of the ferrite phase depends mainly on composition but for a given analysis will increase with the rate of cooling from elevated temperatures. For example, a 20%Cr, 10% Ni alloy cooled through the temperature range of about 2550 to 1850 F will contain more ferrite at room temperature if cooled rapidly as in welding than if cooled slowly as in casting.

Microfissuring and cracking can occur when low temperature melting com pounds of P, S, Si, and Cb or other tramp elements tend to segregate intergranularly thus setting the stage for hot cracking. Since these low melting compounds are much more soluble in ferrite than in austenite, a small amount of ferrite can "absorb" significant quantities of the impurities and reduce segregation to grain boundaries. With relatively small amounts of ferrite these boundaries are discontinuous, thus interrupting any cracking paths. Therefore resistance to cracking can be enhanced by inducing the formation of ferrite in what might otherwise be a fully austenitic weld deposit.

FRIGHTFUL FERRITE?

Is ferrite always desirable? Ferrite can get you into trouble in some areas. In molybdenum bearing grades like 316 and 317 and their L versions, ferrite can cause a major decrease in corrosin resistance in hot oxidizing media ( e.g. urea) that's why some stainless steel filler metal manufcturers offer a 316L grade with a typical ferrite number (FN) of 1 in addition to a 316L HF with a typical FN of around 8 specified for most applications.

In cryogenic applications, the low temperature impact strength decreases with increasing ferrite. A closely balanced FN is required to ensure adequate crack resistance and good low temperature impact strength. Above about 10 FN there is danger of ferrite (which contains some Cr in solid solution) transforming to brittle sigma phase when the service temperature is in the 1000 - 1650 F range. In some multi-pass welds the exposure to the heat of welding alone may produce sigma phase in high ferrite welds. For elevated temperature service 309L with a relatively low FN would be preferred. The effect of adding Mo to the latter would promote sigma and some chi phases at elevated temperatures but would increase crack resistance at normal temperatures.

Here's a very good article from the AWS & The Welding Research Council which was published in the May 2006 edition of the AWS Journal... It basically talks about prediction of ferrite number of Duplex stainless steel clad metals using RSM (Response Surface Methodology):

http://files.aws.org/wj/supplement/wj0506-91.pdf

Here's another one which talks about how ferrite numbers between 3 and 8 but usually around 5 and above which leads to welds with better vacuum integrity and increased ferrite can improve resistance to stress corrosion.

http://www.mksinst.com/docs/ur/tubing.pdf

This is a paper on the most current method of predicting ferrite numbers in stainless steels:

http://www.igcar.ernet.in/benchmark/science/4-sci.pdf

Here is a ferrite number prediction calculator one can use online:

http://web.archive.org/web/20050402230743/http://engm01.ms.ornl.gov/FNPLots.HTML

Another good article which the above poste FN prediction calculator came from:

http://www.sci.fi/~benefon/ferritedoc.html

More good "stuff" from J Jlammi:

http://www.sci.fi/~benefon/Metallurgy.php

This one a very interesting article from the Department of Energy's OSTI Bridge website:

http://www.osti.gov/bridge/purl.cover.jsp?purl=/14580-mGkMzM/webviewable/

This one is one of my favorites:

http://www.msm.cam.ac.uk/phasetrans/2005/Stainless_steels/stainless.html

As is this also:

http://www.msm.cam.ac.uk/phase-trans/2001/Ferrite_number.pdf

And finally this beautiful piece of Genius:

http://www.msm.cam.ac.uk/phase-trans/2001/ferrite.number.html/index.html

Now if you just want something that's fundamental and straight to the point regarding Delta ferrite, Carbon ,P, S, and any other alloying elements found in a variety of Stainless steels, then I've got an article for you!!! The title is: "The Role of Alloying Elements in the Fabricability of Austenitic Stainless Steel" and here it is:

http://www.csidesigns.com/tech/fabtech.pdf

Another article which also briefly covers Magnetic Permeability and the various methods of measuring ferrite in actual welds:

http://www.stainlessfoundry.com/LowFerrite.asp

This one is for anyone interested in the influence of heat input on Austenite in submerged arc weld metal, and how it influences weld metal ferrite content, and impact properties of -196 C.

http://www.crown.co.za/support/AFSep09/Heatinput.pdf

Another interesting article form ORNL (Oak Ridge National Laboratories)

http://www.ornl.gov/~webworks/cppr/y2001/pres/113776.pdf

And finally this one from the Corrosion doctors:

http://corrosion-doctors.org/MatSelect/corrstainsteel.htm

I believe the "csidesigns" one ( http://www.csidesigns.com/tech/fabtech.pdf )
will satisfy some of the questions you may have, and then if you want to explore further, you can venture into the other links I posted! ;)
Enjoy the reading!!! :) :) :)

Respectfully,
Henry

By G.S.Crisi (****)

Date 11-23-2009 11:12

Very good, Henry, I would like to include a drawing showing how grain boundaries form, which would illustrate what you've explained. Problem is that I don't know how to paste a figure to my posting (I should know, agree, but nobody's perfect).
Giovanni S. Crisi
Sao Paulo - Brazil

By 3.2 Inspector (***)

Date 11-23-2009 11:54

http://www.aws.org/cgi-bin/mwf/forum_help.pl

3.2

EDIT: After you submits your post, you can edit it with an attachment.

By G.S.Crisi (****)

Date 11-27-2009 18:58

The two sketches below illustrates clearly what Henry has said on his postings.
The first one shows how solidification of a metal (iron in this case) takes place. The tiny squares in the sketch represent the FCC unit cells of austenite, if carbon content is higher than 0.008%, or the ferrite BCC unit cells of ferrite if it's lower. Take into account that at the solidification temperature of iron, when the carbon content is higher than 0,008%, the stable alothropic form of iron is austenite, as the Fe - C diagram shows.
The second one shows how the iron grains look like if we were able to see them in a microscope, which up to this moment we can't.
Giovanni S. Crisi

Attachment: Solidificationsequence.doc (281k)

Attachment: Grainstructure.doc (281k)

By G.S.Crisi (****)

Date 11-23-2009 11:06

Very interesting, Henry, author Carl G. Johnson doesn't define ferrite as being the BCC (body centered cube) alothropic state of iron; and this for the simple reason that in 1956 (when I was a college student) it wasn't still known that ferrite structure was BCC.
Giovanni S. Crisi