Cast Stainless Steel Technology Developments

Home , Casting (metalworking)

Raymond Monroe SFSA

CN3MN

CF8 CD4Cu CB 7 Cu CA 15

Calculation of Chromium Equivalent and Nickel Equivalent

CrE = %Cr + 2 × %Si + 1.5 × %Mo + 5 × %V

NiE = %Ni + 0.5 × %Mn + 30 × %C + 0.3 × %Cu Ferrite from Chemistry

Schoefer Diagram

2.2

2.1 • Chemistry: C=0.07, 2 Mn=0.56, Si=1.30, 1.9 P=0.028, S=0.009, 1.8 o i t 1.7 Cr=19.5, Ni=10.7, a

1.6 on R i t i Mo=2.18 (Cb ~ 0.05 1.5 pos m

o and N ~ 0.04) 1.4 C i N / r 1.3 C • ASTM A800 predicts 1.2 10.5 volume percent 1.1

1 ferrite with a range of

0.9 Cr Cr (%) + 1.5Si (%) + 1.4 Mo (%) + Nb (%) − 4.99 e = 6.5 to 14.5 (chromium Ni e Ni (%) + 30 C (%) + 0.5Mn (%) + 26 ( N − 0.02 %) + 2.77 0.8 010203040506070 equivalent to nickel Volume Percent Ferrite equivalent = 1.19) Means of Calculating Ferrite • Severn Gage: 11 • Feritscope: 7 • Magne-Gage: 2 • Two different instruments: 5 • Manual point count • ASTM A800 • 1949 Schaeffler Diagram • WRC Diagram

Unit of measure: • FN: 8 (all 4 of the non- foundry) • Volume percent: 7 • Use both methods: 2 Identification of Phases by Composition

FERRITE

AUSTENITE Stainless Steels - Strength

Grade Yield (ksi) UTS (ksi)

CF8 70 30

CF3MN 75 37

4A(2205) 90 60

6A(Zeron 100) 100 65 Stainless Steel - Corrosion

Grade Critical pitting temperature oC CF8 5 (calculated)

CF3MN 29 (calculated)

4A(2205) 35 - 40

6A(Zeron100) 45 – 55 Pseudo Phase Diagram for 68 % Fe – Cr Ni

CCT Diagram - CD3MN 5oC/min 2oC/min 1oC/min 0.5oC/min 0.1oC/min 0.01oC/min

1100

1050 TTT curves 1000 (initial & final) 950

900

850

800

750

700 Temp. (Celsius) 650 CCT curves 600 none (non-equil. 550 initial & final) 500 1 10 100 1000 10000 100000 Time (minutes)

Cooling Rate=1oC/min, Cooling Rate=0.5oC/min Cooling Rate=0.1oC/min Cooling Rate=0.01oC/min σ phase <<1% σ phase <1% σ phase =8.72% ±3.38 σ phase =17.62% ±3.46 CCT Diagram - CD3MWCuN V0.175mm/sec V0.150mm/sec V0.100mm/sec 5oC/min 2oC/min 1oC/min 0.5oC/min 0.1oC/min 1100

1050

1000

950 TTT curves (initial & final)

) 900

us 850 si o 800 Cooling Rate=0.1 C/min σ phase =10.60% ±2.82 (Cel 750 mp. 700 Te 650 none 600 CCT curves (non-equil. 550 initial & final) 500 1 10 100 1000 Time (minutes) Cooling Rate=0.5oC/min σ phase =10.37% ±2.22

Bridgman furnace Cooling Rate=5oC/min Cooling Rate=2oC/min Cooling Rate=1oC/min V=0.1mm/sec,σ phase<1% σ phase =3.61% ±2.64 σ phase =7.10% ±2.12 σ phase =10.09% ±1.13 PROBLEM - Corrosion of High Alloy (6 wt% Mo) Stainless Steel Castings

Wrought AL6XN ACN3MNs-Solidified AL6XN 1150oC/4 hrs

Current minimum required heat treatment – 1150 °C/ 1hr (ASTM Standards A743 10 Dendrite cores Mo and A351) 8 Cast alloys and welds not as corrosion resistant due to: 6 – microsegregation element – presence of σ phase 4

Wt % of 2 OBJECTIVE: Develop heat treating schedules and welding procedures that restore the corrosion resistance of castings 0 and welds to a level comparable to that of wrought counterpart alloys 010 20 30 40 Distance (µm) Typical Homogenization Results

1205 4 H our CK3M CuN 10 1150As Ca 4 Hst oCKur 3CMKCu3MNC uN 10 10 % Mo t 9 De n drite Co re s Mo

w Mo 9 ) w 9 n ( % o t i n ( t w 8 o i a t 8 n ( r 8 a o t i r t n a e r 7 t

n 7 nc e 7 ncent o o nc o %) C 6 C C 6 6 num num 55 bde 5 y bdenum l bde y o y l l M o

o 44

M 4 M

33 3 0 0 0 2020 5 0 4040 6061000 8080 150 100100 120120200 141400 250 161600 181803000 Di s tanc e ( m i c r o ns ) DDiissttaanncece ((mMicicroronns)s)

• 1205 °C/4 hours needed for complete homogenization CN3MN Corrosion Results – ASTM G48A 25.0% 22.3%

) 20.0% 18.0% 18.3% % ( s 15.0% 13.8% 13.2% Los 10.0% ght i e 5.4% ASTM Minimum 5.0%

W 5.0% 1.7%

0.0% AL6XN As Cast 1150 1 Hour 1150 2 Hour 1150 4 Hour 1205 1 Hour 1205 2 Hour 1205 4 Hour CN3MN λ = 80ε −0.33 Model can be used as a predictive tool to determine effective heat treatment times for CN3MN prepared with various cooling rates Need to establish acceptable cooling rate from the heat treating temperature to avoid formation of brittle secondary phase Collaboration with Scott Chumbley (Iowa State)

Results from C. Muller and Scott Chumbley, Iowa State University

Impact toughness

Impact Toughness (ft-lbs) Impact Toughness Grain boundary precipitates

Impact toughness decreases significantly with time at 870 oC, which could occur for large castings cooled slowly from the heat treating temperature Need to establish critical cooling rates in order to avoid embrittlement. Toughness and Corrosion CK3MCuN CN3MN Performance

Charpy Impact Toughness decreases significantly with an extremely slow cooling rate (0.01°C/sec) Charpy Impact Toughness is unaffected by cooling rate above 1°C/sec

Corrosion Performance decreases at an extremely slow cooling rate (0.01°C/sec) CK3MCuN CN3MN Corrosion Performance is unaffected by cooling rate above 1°C/sec Slow cooling rate sample shows evidence of grain boundary attack Corrosion of Welds in Super Austenitic Stainless Steel Castings CN3MN CN3MN CN3MN 1205oC/4 hrs with As-Cast 1205oC/4 hrs Autogenous Weld

Base 1 cm Metal Weld

Welding reintroduces the microsegregation profile 1 cm 1 cm

Percent Mass Loss Percent Mass Loss CN3MN CN3MN CN3MN 1205°C/4hr As-Cast 1205°C/4hr with Autogenous Weld 22% 2% 14% Corrosion Resistance as a Function of Dilution (IN686 Filler Metal Welds on CN3MN)

ASTM G48a Temperature: 75° C Solution: FeCl3 (Ferric Chloride)

1 cm 1cm 1cm 1cm

6.2 wt% Mo

Weld Metal Base Metal 7.9 wt% Mo 1cm

11.0 wt% Mo

14.0 12.4 wt% Mo wt% Mo

Corrosion performance increases with decreasing dilution level Corrosion performance of weld can restored to level of cast material at dilution levels below ~ 50% Corrosion Resistance as a Function of Dilution (IN72 Filler Metal Welds on CN3MN)

ASTM G48a Temperature: 75° C Solution: FeCl3 (Ferric Chloride) 1 cm 1 cm 3.2 wt% Mo 6.2 wt% Mo 31.5 wt% Cr 5.1 wt% Mo 20.6 wt% Cr 24.4 wt% Cr

Base Weld Metal Metal

1 cm 1 cm 1cm

1.3 wt% Mo 2.3 wt% Mo 38.5 wt% Cr 34.9 wt% Cr

Corrosion performance increases with decreasing dilution level IN72 Filler metal unable to avoid localized corrosion in weld Corrosion Results: IN686 Filler Metal

Heat Treatments have a significant effect on corrosion performance Post weld heat treatments (PWHT) will mitigate negative effects of dilution Heat treatments should be done after welding, if possible (no need to control dilution) Post Weld Heat Treatment at 1205°C/4hr produces the best corrosion resistance Air-Set Pouring Nondestructive Examination

•Surface –Visual – Magnetic particle examination (MT) – Liquid penetrant examination (PT) • Volume – Radiographic examination (RT) – Ultrasonic examination (UT) Examination •Visual – ASTM A802 – MSS SP55 • Magnetic Particle – ASTM E709 – ASTM E125 – MSS SP53 • Liquid Penetrant – ASTM E165 Method – ASTM E433 Acceptance – MSS SP93 Visual Examination

• Equipment required: surface comparator, pocket rule, straight edge, workmanship standards • Enables detection of: surface flaws – cracks, porosity, slag inclusions, adhering sand, scale, etc. • Advantages: low cost, can be applied while work is in process to permit correction of faults • Limitations: applicable to surface defects only, provides no permanent record • Remarks: should always be the primary method of inspection, no matter what other techniques are required Inspection 1 Inspection 2 erator 1 p O

Casting 3 erator 2 p O Inspection 1 Inspection 2 erator 1 p O

Foundry 3 erator 2 p O Liquid Penetrant Examination

• Equipment required: commercial kits containing fluorescent or dye penetrants and developers, application equipment for the developer, a source of ultraviolet light – if fluorescent method is used • Enables detection of: surface discontinuities not readily visible to the unaided eye • Advantages: applicable to magnetic and nonmagnetic materials, easy to use, low cost • Limitations: only surface discontinuities are detectable Methodology

• To determine the resolution of the process – Only data from a minimum of three inspections per casting for each inspector was used. – An indication must be detected at least 50% of the time or the data was not considered so results are a best case situation. • The results were grouped by casting type, if multiple geometries were used, and also by inspector (to eliminate variation from inspector to inspector). • Sample standard deviation was used as a measure of resolution. Foundry 3

• High alloy steel, visible liquid penetrant. • One casting shape, 10-15 lb cast weight. • Three Inspectors. • Twenty-four pieces per part type. • Three runs per inspector • Measured length and sketched location of indications on part drawing. • A total of 216 inspections were made. Casting Geometry 1 Inspector 1 Inspector 2 Inspector 3 All Box-and-Whisker Plot Box-an d-W hisker Plot Box-and-Whisker Plot Box-and-Whisker Plot

0.009 0.013 0.017 0.021 0.025 0 0.1 0.2 0.3 0.4 0 0.005 0.01 0.015 0.02 0.025 00.10.20.30.4 Standard Deviation Standard Deviation Standard Deviation Standard Deviation 95% confidence interval 95% confidence interval 95% confidence interval 95% confidence interval for mean for mean for mean for mean 0.0” to 0.04” 0.0” to 0.2” 0.0” to 0.02” 0.01” to 0.5”

Punch Line – The resolution for all inspectors is about 0.4”. Radiographic Examination • Equipment required: commercial x-ray or gamma units made especially for inspecting welds, castings, and forgings with film and processing facilities • Enables detection of: internal macroscopic flaws – cracks, porosity, blow holes, non-metallic inclusions, shrinkage, etc. • Advantages: when the indications are recorded on film, gives a permanent record • Limitations: cracks difficult to detect, requires safety precautions, requires skill in choosing angles of exposure, operating equipment, and interpreting indications • Remarks: radiographic inspection is required by many codes and specifications, useful in qualification of processes, its use should be limited to those areas where other methods will not provide the assurance required because of cost Results for Unanimous Agreement in X-Ray Ratings • Unanimous agreement in shrinkage type: 37% (47/128) • 14 Level 0 (no type) • 20 CB •7 CA •6 CC • Unanimous agreement in shrinkage level: 17% (22/128) • 14 Level 0 • 1 Level 1 • 1 Level 2 • 1 Level 3 • 5 Level 5 • Unanimous agreement in level and type: 12.5% (16/128) • 14 Level 0 •2 CB5 Casting Simulation – Each trial plate was simulated with recorded casting conditions – Niyama values (Ny) were measured •minimum Ny • area of Ny < 0.1 (K-s)1/2mm-1 top view cross-section

G Ny = T& G: temperature gradient (K/mm) side view cross-section T: cooling rate (K/s) Casting Simulation – Combine trial and simulation results: 6 W/T = 2, CF-8M (20 Plates) Mean Minimum Niyama Values • Trial results: X-ray level vs. FL +/- One Standard Deviation W/T = 5.5, CF-8M (50 Plates) 6 5 W/T = 8, CF-8M (30 Plates) • Simulation of trials: Nymin vs. FL5 W/T = 8, HH (20 Plates) 25 0.005 • CombineW/T = 8,by HP eliminating (20 Plates) FL →4 X-ray level vs. Ny 0.022 min 4 6 W/T = 12, CF-8M (25 Plates) 3 0.024 2 Level 5: 25 Plates 0.074

Level 4: 6 Plates X-ray Level 3 1 Level 3: 12 Plates 0 ge X-ray Level 11 Level 2: 15 Plates a Level 1: 24 Plates -1 Level 0: 83 Plates 2 00.10.2 14 TOTAL: 165 Plates 1/2 1/2 -1 Nymin (K s mm )

1 ASTM Shrink 15 9

13 70 -1 -0.1 0 0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 1/2 1/2 -1 Minimum Niyama Criterion Value, Nymin (K s mm ) Variation in the Ratings: Confidence intervals of x-ray level ratings grouped by average x-ray level

2.5

Data from all foundries Average one-sided student-t 95% 27 x-rays confidence interval for all 128 x-rays = 1.42 2.0 11 x-rays

35 x-rays 8 x-rays

1.5

1.0

39 x-rays

0.5

Xavg < 3.5 Xavg < 4.5 8 x-rays Xavg < 1.5 Xavg < 2.5 2.5 < 3.5 < 4.5 1.5 < 0.5 < Average One-Sided Student-t 95% Confidence Interval Average One-Sided Student-t Level < 0.5 Average X-ray

0.0 Xavg > 012345 Average X-ray Level Groupings Background • The root cause of leaks in fluid-containing castings can be shrinkage porosity that extends through a wall. • Sometimes, porosity is so small that it cannot be detected using industrial radiography. • No method available to assure quality. Background • The Niyama criterion, a common output from a casting computer simulation, can be used to predict shrinkage porosity.

Ny = G T& Background • What is the critical Niyama below which shrinkage porosity forms? (especially for high-Ni alloys)

o Micro-porosity, Nymicro (not visible on radiograph)

o Macro-porosity, Nymacro (visible on radiograph)

Pore Volume (%)

macro-shrinkage

micro-shrinkage Pore Volume (%) 0

log10(Ny)

Nymacro Nymicro Ny = G T& Case Study #3: Valve B

Casting defect. Leaks around gasket • CG8M steel valve cast in a silica sand outer diameter shell mold of flange = 5”

• After machining, 22% leaked around gasket on flange face during pressure testing

• No shrinkage visible on x-rays Case Study #3: Valve B

∼ 0.1 mm • leaking castings were sectioned, and photomicrographs were taken

• microporosity evident in leaking area Case Study #3: Valve B original rigging revised rigging

padding added

chill setup changed Case Study #3: Valve B revised rigging flange face Ny = 1.36 Ny = 1.38

diameter as-cast

machined diameter

outer diameter of gasket seat

Nymin =1.4 Case Study #3: Valve B

• Microporosity caused leaks in original rigging with Nymin = 0.5 - 0.7

• Nymin = 1.4 for revised rigging • Valves cast since rigging revised: • none have leaked • none have had any shrinkage indications on x-ray Leaker Case Study #1

• Leaker was a 20# investment cast M35-1 valve leak • 8 valves were cast and shipped to the customer, and one leaked during the customer’s pressure testing • Leaker was returned to the foundry, with leak area circled • Metallographic analysis (John Griffin, UAB) Photographs of Defects in Leak Area • Outer diameter (OD) of leak area: 1 mm leak exit location

18 mm below mid-plane mid-plane

mid-plane 18 mm below mid-plane Photographs of Defects in Leak Area • Casting through-thickness toward inner diameter (ID) of leak area: Photographs of Defects in Leak Area • Polished through-thickness specimen toward ID of leak area, showing shrinkage porosity: Niyama Values in Entire Valve Niyama Values in Region of Leak

Nymin = 0.58

Nymin = 0.69

mid-plane mid-plane

Niyama values are in units of (°C-s)1/2/mm Niyama Values in Region of Leak

photos and simulation results OD are from the valve mid-plane Case Study #1 Summary

• Simulation suggests potential for leak in valve – At valve mid-plane: 1/2 Nymin = 0.58 (°C-s) /mm at ID, Nymin = 0.69 at OD

• These low Niyama values (Nymin < 1) correspond well with shrinkage porosity observed in metallographic sections. Additional Examination Areas • After examining the leaking area, additional areas were selected for metallographic examination (all regions on valve mid-plane):

3 3 2 6 2 1 6 1

5 4 5

4 region of leak Comparison: Region #1 Comparison: Region #2

A B A

B Comparison: Region #3

B A B

A Comparison: Region #4 Comparison: Region #5

A C

D C B B D

C A

B D Comparison: Region #6 Additional Region Summary

• In general, areas with Ny < 1 show a significant amount of porosity (macroporosity) • In general, areas with 1 < Ny < 2 show a noticeable amount of microporosity – Region #4 was a slight exception, with no substantial microporosity in a region with 1.5 < Ny < 1.8 • In general, areas with Ny > 2 were free from shrinkage porosity Leaker Case Study #2 • 150# CN7M Valve Sand Casting:

leak (outside) leak (inside) Leaker Case Study #2

cross-section of leaking area

shrinkage, possibly segregation Case Study #2 Simulations • Original rigging:

• Nymin = 1.6 at OD, Nymin = 0.5 at ID • visible macro-porosity for Nymin < 1.0 Conclusions

• In general, areas with Ny < 1 show a significant amount of porosity (macroporosity) • In general, areas with 1 < Ny < 2 show a noticeable amount of microporosity • In general, areas with Ny > 2 were free from shrinkage porosity • It appears that the Niyama criterion can be used for quality assurance, especially if shrinkage porosity is so small that it cannot be detected by radiography. • Note for MAGMAsoft users: the valves had only minimal feeding indications (porosity %); the shrink seen in the valves was only predicted using Niyama criterion.