25. Pseudobinary Phase Diagram @ 70% Iron
Let us just take a few minutes to examine the various grades of stainless steel and how the chemistry effects which grade is present. This can best be seen with the pseudobinary phase diagram between Cr and Ni at a constant 70% Iron. Note that at High Ni (low Cr), when the alloy solidifies it will form austenite and this phase will be present to room temperature, thus the type A or austenitic stainless steel will be formed. At slightly higher Cr (lower Ni) a eutectic is present and the material at first solidifies as austenite but then begins to form some amount of ferrite at the dendrite boundaries and the phase transformation continues. This is the AF type and represents the bulk of the austenitic grades of stainless. As we will see later, the amount of ferrite present can effect the weldability. At still higher levels of Cr, the FA grades form. These grades solidify as primary ferrite and then undergo an austenite transformation at lower temperatures. Finally at very high Cr (low Ni) the material solidifies as delta ferrite and remains ferrite (type F).
Pseudobinary
Phase Diagram
@ 70% Iron
AWS Welding Handbook

26. Prediction of Weld Metal Solidification Morphology Schaeffler Diagram
The Schaeffler and WRC diagrams are used to predict the amount of ferrite present (primarily in the AF – austenite/Ferrite). The diagrams are a plot to the chromium vs. Nickel equivalents, which are the relative potency to form ferrite or austenite as described above. Note the regions on this diagram typical of the chemistries of each of these stainless steel grades. These diagrams can also be used to predict the amount of ferrite in welds made between two (or more) different grades of stainless steels by considering the dilution of each in the weld metal. As a general rule, the presence of about 5 % ferrite or greater is needed in the austenitic weld metal in order to avoid hot cracking during that weld solidification.
WRC
Diagram
AWS Welding Handbook

27. Hot Cracking P+S A few % Ferrite Reduces Cracks
But P&S Increase Cracks
These diagrams show the hot cracking observed in these austenitic stainless steels. With low relative levels of Cr equivalent (a Cr/Ni ratio of about 1.0), the material solidifies as austenite and cracking occurs. With higher levels of Cr equivalent, more ferrite forms and a dramatic reduction in hot cracking occurs. Even in this region of crack sensitivity, it is seen that with extremely low levels of sulfur and phosphorous, some crack improvement is noted but with commercial levels of these elements, the Cr/Ni ratio of 1.5 is seen to be very important for crack reduction. Above 1.5% there is no cracking. So there are two concerns. Both solidification morphology and the presence of S & P are important.
AWS Welding Handbook

28. Spot Welding Austenitic Stainless Steel
Some Solidification Porosity Can Occur:
As a result of this tendency to Hot Crack when Proper
Percent Ferrite is not Obtained
Because of higher Contraction on Cooling
As a result of the discussion on the previous couple of slides, we can see that some centerline solidification cracking or shrinkage porosity is common in the austenitic stainless steels. This is even more prevalent when a sufficient amount of ferrite in the weld nugget is not provided. The greater amount of thermal contraction of the stainless steels when compared to plain carbon steels also results in this being a greater problem. To counteract this, the maintenance of electrode force until the weld is fully cooled is common practice, thus slightly higher hold times occur. In some cases a force force during the solidification time might also help.. The nugget diameter is limited to less than 4 times the thickness of the thinner piece. Larger weld nuggets result in more solidification shrinkage. Thus more smaller diameter spot welds are preferred to fewer large welds.
Suggestions:
Maintain Electrode Force until Cooled
Limit Nugget Diameter to <4 X Thickness of thinner piece
More small diameter spots preferred to fewer Large Spots

29. Spot Welding Austenitic Stainless Steel
Some Discoloration May Occur Around Spot Weld
Oxide Formation in HAZ
Nugget
Solutions
Maintain Electrode Force until weld cooled below oxidizing Temperature
Post weld clean with 10% Nitric, 2% Hydrofluoric Acid (Hydrochloric acid should be avoided due to chloride ion stress-corrosion cracking and pitting)
A second problem which may occur is discoloration around the spot weld on the top and bottom surfaces. This comes from the formation of oxides as a result of the heat affected zone around the weld. Again longer hold times to increase cooling and lower the temperature around the weld can reduce the oxidation, and a post weld acid cleaning can remove any oxide that forms..

30. Knifeline Corrosion Attack in Austenitic Stainless Steel Seam Welds
Seam Welding Austenitic Stainless Steel
Somewhat more Distortion Noted Because of Higher Thermal Contraction
Solution
Abundant water cooling to remove heat
Knifeline Corrosion Attack in
Austenitic Stainless Steel Seam Welds
In seam welding, a larger total volume of nugget is formed. The increase in thermal contraction experienced in the stainless steels will naturally result in more distortion than observed in plain carbon steel seam welds. Abundant water flow to remove the heat of weld can help. A second problem that occurs is that of knifeline corrosion attack.
Solution
See Next Slide for more description

31. Chromium Carbide Precipitation Kinetics Diagram
1500 °F
1500 F
M23C6
Precipitation
1200 °F
800 F
Temperature
Chromium Oxide
800 °F
All of the austenitic grades contain carbon in solid solution. As these steels are heated into the 800 to 1500 °F temperature range, the kinetics for chromium carbide precipitation increase as shown in the transformation diagram in the above slide. These carbides form at the grain boundaries making the material more sensitive to inter-granular corrosion. Therefore, welding conditions must be adjusted to minimize carbide formation. Preventative measures that can be taken are also listed in the above slide.
When seam welding the austenitic grades, care must be taken to avoid a “knife-line” attack which can be produced throughout the weld area.
M23C6
Chromium-Rich
Carbides
Intergranular
Corrosion
Time

32. Lower carbon content in the base material 304L, 316L
Preventative Measures
Short weld times
Low heat input
Lower carbon content in the base material
304L, 316L
Stabilization of the material with titanium additions
321 (5xC)
Stabilization with columbium or tantalum additions
347, 348 (10xC)
Lower nitrogen content (N acts like C)
Ways to reduce or prevent knife line attack are list here. Short weld times and low heat inputs result in very narrow heat affected zones where this chromium carbide precipitation is limited. Lowering the carbon obviously reduces the carbon present for carbide precipitation. Stabilizing with more active carbide formers like titanium columbium and tantalum cause these carbides to form leaving the chromium to form the beneficial oxide. Finally lower the nitrogen seems to help as it acts very much like carbon in the formation of carbo-nitrides.

33. Projection Welding Austenitic Stainless Steel
Because of the Greater Thermal Expansion and Contraction, Head Follow-up is critical
Solution
Press Type machines with low inertia heads
Air operated for faster action
In Welding Tubes to tube sheets with Ring projections for leak tight application, electrode set-up is critical
Projection welding of austenitic stainless steels has been performed as well. There are some concerns, however as illustrated here. Welding equipment with good head follow-up (I.e. low inertia heads) are necessary for all projection welding but it is even more critical with the austenitic stainless steels as they have greater thermal expansion and contraction than do the plain carbon steels. When tubes are welded to tube sheets and the weld ring thus produced are required to be leak tight, the alignment of the electrodes is very critical.
Solution
Test electrode alignment

34. Cross Wire Welding Austenitic Stainless Steel
Often used for grates, shelves, baskets, etc.
Use flat faced electrodes, or
V-grooved electrodes to hold wires in a fixture
As many as 40 welds made at one time
Cross wire projection welding is often used for grates and shelves in grill or oven applications and baskets in heat treating and other applications. Recommendations for welds in these applications are presented here.

35. Flash Welding Austenitic Stainless Steel
Current about 15% less than for plain carbon
Higher upset pressure
The higher upset requires 40-50% higher clamp force
Larger upset to extrude oxides out
Flash weld have also been made in austenitic stainless steels. Because of the higher electrical resistance and lower thermal conductivity, lower current can be used. Because of the higher hot strength when compared to p;lain carbon steels, higher upset pressures are needed. This also then necessitates higher clamping forces to prevent grip slipping during upset, and larger upset distances are needed to extrude the oxides out.

36. Super Austenitic
Alloys with composition between standard 300 Austenitic SS and Ni-base Alloys
High Ni, High Mo
Ni & Mo- Improved chloride induced Stress Corrosion Cracking
Used in
Sea water application where regular austenitics suffer pitting, crevice and SCC
The super austenitic stainless steels are alloys with compositions between standard 300 austenitic SS and NI-base alloys with high levels of Nickel and Molybdenum. These steels are used in chloride environments because of their stress corrosion cracking resistance.

37. This is a table of the chemistry of the super austenitic stainless steels.
AWS Welding Handbook

38. Copper and Copper Alloy Electrodes can cause cracking:
The Super Austenitic Stainless Steels are susceptible to copper contamination cracking. RESISTANCE WELDING NOT NORMALLY PERFORMED
Copper and Copper Alloy Electrodes can cause cracking:
Flame spray coated electrodes
Low heat
These super austenitic stainless steels are susceptible to copper contamination cracking. Copper on the surface of these materials infiltrates the grain boundaries and causes cracking. As a result, they are not usually resistance welded as the resistance welding electrodes are of necessity usually made from copper base alloys. When resistance welding must be performed, electrodes with a protective (diffusion barrier) coating must be used. These includes an aluminum oxide and/or silver flame sprayed coatings. And the weld heat is kept as low as possible to reduce copper alloying and contamination.

39. Nitrogen-Strengthened Austenitic
High nitrogen levels, combined with higher manganese content, help to increase the strength level of the material
Consider a postweld heat treatment for an optimum corrosion resistance
The nitrogen strengthened austenitic stainless steels have high nitrogen and manganese contents. These help strengthen the material and replace some of the higher cost Cr. Not much weld data is available but these should be able to be welded similar to the standard austenitics. Because of the high nitrogen they may suffer from the knifeline attack mentioned previously for the austenitic stainless steels. The consideration of a post weld heat treatment for optimum corrosion resistance is recommended.
Little Weld Data Available

40. Martensitic
Contain from 12 to 18 percent chromium and 0.12 to 1.20 percent carbon with low nickel content
Combined carbon and chromium content gives these steels high hardenability
Magnetic
Tempering of the low-carbon martensitic stainless steels should avoid the 440 to 540 °C temperature range because of a sharp reduction in notch-impact resistance
Martensitic stainless steels contain from 12 to 18 percent chromium and 0.12 to 1.20 percent carbon. These steels can be heated to form austenite or austenite plus carbide then cooled to form martensite. These steels invariably fall within the alpha loop of the phase diagram, defined by the relation: %Cr - 17(%C) The combined carbon and chromium content gives these steels such high hardenability that they can be air-hardened in large sections.
Tempering of the low-carbon martensitic stainless steels should avoid the 440 to 540 °C temperature range because of a sharp reduction in notch-impact resistance. This is accompanied by a small secondary hardening peak. This temper embrittlement can also increase stress corrosion cracking.
Applications:
Some Aircraft & Rocket Applications
Cutlery

41. Martensitic SS Wrought Alloys are divided into two groups
This is a table of chemistries of the martensitic stainless steels. Note the low nickel content. The lower carbon grades on top are the engineering (structural) grades, while the higher carbon grades (in the middle) are the hard grades used for cutlery.
Martensitic SS Wrought Alloys are divided into two groups
12% Cr, low-carbon engineering grades (top group)
High Cr, High C Cutlery grades (middle group)
AWS Welding Handbook

42. From a Metallurgical Standpoint, Martensitic SS
is similar to Plain Carbon
(12% Chromium)
The martensitic steels behave much like the plain carbon steels. This is a pseudo binary diagram at the 12% chromium level. Note the size of the austenite phase region. In the lower diagram, a comparison of the size of this austenite region varies as chrome content varies. As the chromium increases this region gets smaller, so there is a limit to the amount of Cr that can be added. Upon quenching from this austenite, martensite is formed as illustrated on the CCT diagram on the right.
AWS Welding Handbook

43. Martensitic Spot Welding HAZ Structural Changes
Tempering of hard martensite at BM side
Quench to hard martensite at WM side
Likelihood of cracking in HAZ increases with Carbon
Pre-heat, post-heat, tempering helps
Flash Weld
Hard HAZ
Temper in machine
High Cr Steels get oxide entrapment at interface
Precise control of flashing & upset
N or Inert gas shielding
Both spot welding and Flash welding has been performed on these steels. The concern here is with structural changes occurring within the heat affected zone. Tempering occurs on the base metal side of the HAZ but hard quenched martensite occurs on the weld side of the HAZ. As the carbon content increases, the transformation stresses occurring increases and quench cracking becomes a possibility. Preheat, post heat and tempering helps reduce these stresses. In flash welding, the higher Cr grades can also experience entrapment of oxides.

44. Effect of Tempered Martensite on Hardness
As Quenched
Loss of Hardness and Strength
Hardened Martensite
Tempered Martensite
Hardness
Fusion
Zone
SS with carbon content above
0.15% Carbon (431, 440) are
susceptible to cracking and need
Post Weld Heat Treatment
HAZ
When the carbon content is below 0.15 percent, the martensitic grades can be usually welded without major difficulty. However, as the carbon content exceeds 0.15 percent, cracking susceptibility significantly increases. Post weld heat treatment is therefore required. As with other structurally refined steels, the weld thermal cycle can also induce local thermal softening. As shown in the above slide, in a weld produced with no postweld treatment, the soft region observed between the weld and base material can present cracking problems. A postweld treatment tempers the martensite, reduces its hardness, and produces a hardness traverse that does not show as much difference between the weld and base material. This type of condition provides a more favorable weld nugget.
Distance

45. Ferritic
Contain from 11.5 to 27 percent chromium, with additions of manganese and silicon, and occasionally nickel, aluminum, molybdenum or titanium
Ferritic at all temperatures, no phase change, large grain sizes
Non-hardenable by heat treatment
Magnetic (generally)
Ferritic stainless steels contain from 11.5 to 27 percent chromium, with additions of manganese and silicon, and occasionally nickel, aluminum, molybdenum or titanium. The carbon content is kept as low as possible to improve toughness and minimize sensitization that occurs through precipitation of chromium carbides. These steels do not experience any allotropic transformation (I.e. the stay ferritic through all temperature ranges) and thus they are non-hardenable by heat treatment and under welding conditions they can experience large grain growth. They are generally magnetic at all temperatures.
Applications:
Water Tanks in Europe
Storage Tanks

46. This table lists the common grades of ferritic stainless steels.
AWS Welding Handbook

47. FERRITIC STAINLESS STEELS
Spot & Seam Welding
The fact that the ferritic stainless steels do not undergo allotropic phase changes, any heat cycle will tend to cause grain growth. The growth will accentuate with each heat cycle. Thus, in the weld and heat affected zone of spot and seam welds, large grains are expected. This will lead to reduced strength and toughness in these regions.
Because No Phase Change, Get Grain Growth

48. (Decomposition of Iron-Chrome Ferrite)
The larger grain size in the HAZ and Weld metal will reduce both strength and toughness. A form of grain boundary embrittlement occurring around 885 F which comes from the decomposition of iron-chrome ferrite will also cause reduced toughness particularly since the grain sizes are so large. So although these ferritic grades are welded, they must be done so with caution.
(Decomposition of Iron-Chrome Ferrite)

49. FERRITIC STAINLESS STEELS
Flash Weld
Lower Cr can be welded with standard flash weld techniques
loss of toughness, however
Higher Cr get oxidation
Inert gas shield recommended
long flash time & high upset to expel oxides
Flash welds have been made in the ferritic grades. The lower Cr containing grades can be welded with standard plain carbon techniques, but like the spot welds, they suffer from toughness losses. With the higher Cr containing grades, an excessive amount of gummy oxides occur and the use of shielding gas and/or long flash times and higher upset to expel this oxide is often useful.

50. Super Ferritic Lower than ordinary interstitial (C&N) Higher Cr & Mo
Better corrosion (Cr) & Higher Hot Strength (Mo)
The super ferritic stainless steels are ferritic in nature, except they have lower interstitials and higher CR and Mo. So they have even better corrosion resistance and the Mo give these steel improved hot strength.
AWS Welding Handbook

51. Increased Cr & Mo promotes Embrittlement
825F Sigma Phase (FeCr) precipitation embrittlement
885F Embrittlement (decomposition of iron-chromium ferrite)
1560F Chi Phase (Fe36Cr12Mo10) precipitation embrittlement
Because of the Embrittlement,
Resistance Welding is Usually
Not Done on These Steels
The CR and Mo promotes embrittlement. There are three forms of embrittlement of concern. The first occurs at about 825F where sigma phase precipitates (FeCr) precipitate and cause embrittle. The second is at 885F where iron-chromium ferrite decomposes, and the third is at 1560 F where the Chi Phase precipitation causes embrittlement.

52. Precipitation-Hardened
Can produce a matrix structure of either austenite or martensite
Heat treated to form CbC, TiC, AlN, Ni3Al
Possess very high strength levels
Can serve at higher temperature than the martensitic grades
Precipitation-hardened stainless steels can produce a matrix structure of either austenite or martensite. Although the austenitic form is less frequently used, it does have the advantage of being nonmagnetic. The precipitation-hardened alloys are solution treated around 1200 °C, quenched, and aged between 700 and 800 °C to form columbium carbide, titanium carbide, aluminum nitride and nickel aluminide precipitates. These alloys possess very high strength levels and can serve at higher temperatures than the martensitic grades.
Applications:
High Strength Components in Jet & Rocket Engines
Bombs

53. This table lists the most common precipitation hardened stainless steels. Three grades are listed, Austenitic, Semi Austenitic, and Martensitic. The martensitic grades are further divided into moderate and high strength martensitic precipitation hardened stainless steels.
AWS Welding Handbook

54. Solution heat treat above 1900F Cool to form martensite
Martensitic
Solution heat treat above 1900F
Cool to form martensite
Precipitation strengthen
Fabricated
Semiaustenitic
Solution heat treat (still contain 5-20% delta ferrite)
Quench but remain austenitic (Ms below RT)
Fabricate
Harden (austenitize, low temp quench, age)
The martensitc grades are solution heat treated, rapidly cooled to form martensite and then given a precipitation heat treatment to develop the maximum strength. The martensitics are generally welded in this aged condition.
The semi-austenitic grades are also solution heat treated, but even after this treatment they still contain 5-20% delta ferrite thus making them semi-austenitic. They are quenched to room temperature but because the Martensite start temperature is actually below room temperature, they do not form martensite. The semi-austenitics are generally fabricated in this state having austenite and ferrite after fabrication and then they are hardened by heating to fully reaustenize, quenched this time to below room temperature to form martensite and then they are aged.
The austenitic grades remain austenitic at all temperatures. They are either welded and then given a precipitation hardening treatment after fabrication or welded in the aged condition.
Austenitic
Remain austinite
Harden treatment

55. RC=Rapid Cool to RT AC=Air cooled SZC= Rapid cool to -100F
List here are the typical heat treatment conditions for each of these grades of steel as described previously.
RC=Rapid Cool to RT
SZC= Rapid cool to -100F
AC=Air cooled
WQ=Water Quenched
AWS Welding Handbook

56. Effect on Aging on the Nugget Hardness in Precipitation-Hardened Stainless Steels
Aged
Hardness
When Welded in the Aged Condition
Higher Electrode Forces
Post Weld Treatment
Weld schedules similar to those for welding the austenitic grades are used for the precipitation-hardened stainless steels. Since these steels are generally welded in the aged condition, higher electrode forces are required, as well as short weld time. Again, there is commonly a hardness loss in the weld region. This is shown schematically in the hardness traverse of the above slide. In this case, the hardness loss occurs from the dissolution of the strengthening precipitates at the higher temperatures. Postweld treatment is required to restore strength to the weld nugget and relieve any built-up stresses.
Annealed
Weld
Centerline
Distance

57. Precipitation-Hardened
Spot Welding
17-7PH, A-286, PH15-7Mo, AM350 & AM355 have been welded
Generally welded in aged condition, higher forces needed
Time as short as possible
Seam Welding
17-7PH has been welded
Increased electrode force
In summary, the precipitation hardened stainless steels have been spot welded but higher forces and shorter times recommended.
Seam welding has also been done again with increased electrode force. This also results in increased electrode wear.
These steels have also been flash welded but again higher upset pressure has been needed, and then post weld heat treatment done.
Flash Welding
Higher upset pressure
Post weld heat treatment

58. Duplex Low Carbon Mixture: {bcc} Ferrite (over 50%) + {fcc} Austenite
Better SCC and Pitting Resistance than Austenitics
Yield Strengths twice the 300 Series
Early grades had 75-80% Ferrite (poor weld toughness due to ferrite)
Later grades have 50-50
The duplex stainless steels are formulated to have high percentage of ferritic stainless in austenite, with low levels of carbon. The early grades had as high as 80% ferrite but more recent grades are about Because they have both structures they get the benefit of both phases. They have better stress corrosion and pitting corrosion resistance than the pure austenitics, and they have higher yield strengths than the traditional austenitic. The grades with higher ferrite content suffer poor toughness.

59. This table lists the duplex stainless steels and their compositions.
AWS Welding Handbook

60. Sensitive to 885F embrittlement Sigma Phase embrittlement above 1000F
Due to the Ferrite:
Sensitive to 885F embrittlement
Sigma Phase embrittlement above 1000F
High ductile to brittle transition temperatures (low toughness)
Solidifies as ferrite, subsequent ppt of nitrides, carbides which reduces corrosion resistance
Rapid cooling promotes additional ferrite
Not Hot Crack Sensitive
Because of the ferrite phase, they are sensitive to all the embrittlement mechanisms listed previously. But they are not hot crack sensitive.
Resistance welding is generally not recommended.
Resistance Welds generally not recommended because low toughness and low corrosion resistance
Unless post weld solution anneal and quench.

61. Some
Applications

62. Method of Making an Ultra Light Engine Valve
Deep Drawing of Plain Carbon Steel
or Stainless Steel
Stainless Steel Cap
Resistance
Weld
This is a method of making an ultra light popper valve for an internal combustion engine. The valve is formed by cold forming a blank into an elongated cup having an extremely thin wall and a flared open end onto which a cap is welded.
Larson, J & Bonesteel, D “Method of Making an Ultra Light Engine Valve” US Patent 5,619,796 Apr 15, 1997

63. Homework