Cast Stainless Steel Technology Developments Raymond Monroe SFSA
CN3MN
CF8 CD4Cu CA 15
CB 7 Cu
Calculation of Chromium Equivalent and Nickel Equivalent Cr E = %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
• Chemistry: C=0.07, Mn=0.56, Si=1.30, P=0.028, S=0.009, Cr=19.5, Ni=10.7, Mo=2.18 (Cb ~ 0.05 and N ~ 0.04)
2.1 2 1.9 1.8
o i t 1.7 a R n1.6 o i t i s o1.5 p m o C1.4 i N / r 1.3 C 1.2 1.1 1 0.9
Cr e Ni
=
Cr (%) Ni (%)
+
+
1 . 5 Si (%)
30 C (%)
+
+
1 . 4 Mo (%)
0 5 Mn (%)
+
+
Nb (%)
26 ( N
−
4 . 99
0 02 %)
+
2 77
• ASTM A800 predicts 10.5 volume percent ferrite with a range of 6.5 to 14.5 (chromium
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 nonfoundry)
Identification of Phases by Composition FERRITE
AUSTENITE
Stai Stainl nles ess s Stee Steels ls - Stre Streng ngth th 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 o
o
5 C/min
2 C/min
o
1 C/min
o
0.5 C/min
o
0.1 C/min
o
0.01 C/min
1100 1050
TTT curves (initial & final)
1000 950
) s u i s l e C ( . p m e T
900 850 800 750 700 650
CCT curves (non-equil. initial & final)
none
600 550 500 1
10
100
1000
Time (minutes)
10000
100000
CCT Diagram - CD3MWCuN V0.175mm/sec o
5 C/min 1100
V0.150mm/sec o
o
2 C/min
1 C/min
V0.100mm/sec o
0.5 C/min
o
0.1 C/min
1050 1000
TTT curves (initial & final)
950
) s u i s l e C ( . p m e T
900 850
Cooling Rate=0.1 oC/min σ phase =10.60% ±2.82
800 750 700 650
none
600
CCT curves (non-equil. initial & final)
550 500 1
10
Time (minutes)
100
1000
Cooling Rate=0.5 oC/min σ phase =10.37% ±2.22
PROBLEM - Corrosion of High Alloy (6 wt% Mo) Stainless Steel Castings
Wrought AL6XN
AL6XN
CN3MN As-Solidified
1150oC/4 hrs
10
Dendrite cores
t 8 n e m e 6 l e f o % 4 t W 2
Mo
Current minimum required heat treatment – 1150 °C/ 1hr (ASTM Standards A743 and A351) Cast alloys and welds not as corrosion resistant due to: – microsegregation – presence of σ phase
OBJECTIVE: Develop heat treating schedules and welding
Typical Homogenization Results 1205 4 Hour CK3MCuN 10
10
As Cast CK3MCuN 1150 4 Hour CK3MCuN
10
t 9 w ( 9 ) 9 n ( % o 8 t n i t w o ( i a t n r 8 8 a o t i r t t n a e n r t c 77 e n c n 7 e n c o ) o n o % C C C 66 m 6 m m u u u n n n e e e 55 d d b d 5 y b l b y o y l l M o o 44 4 M M 33 3 00
Mo Mo Mo
Dendrite Cores
0
20 20
50
40 40
60 60 100
80 80
150
100 100
Distance (microns)
120 120 200
140 140
Distance (Microns) Distance (microns)
• 1205 °C/4 hours needed for complete homogenization
250
160 160
180 180 300
CN3MN Corrosion Results – ASTM G48A 25.0%
) 20.0% % ( s s 15.0% o L t h 10.0% g i e W 5.0%
22.3% 18.3%
18.0% m u m i n i M M T S A
5.4%
13.8%
13.2%
5.0% 1.7%
0.0% AL6XN
A C
1150 1 Ho
1150 2 Ho
1150 4 Ho
1205 1 Ho
1205 2 Ho
1205 4 Ho
λ =
80ε
−
0.33
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
) s b l t f ( s s e n h g u o T t c a p m I
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
CK3MCuN
CN3MN
Toughness and Corrosion 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 As-Cast
CN3MN 1205oC/4 hrs
CN3MN 1205oC/4 hrs with Autogenous Weld
Base Metal
1 cm Weld
Welding reintroduces the microsegregation profile 1 cm
1 cm
Percent Mass Loss
CN3MN
CN3MN
Percent Mass Loss
CN3MN 1205°C/4hr
Corrosion Resistance as a Function of Dilution (IN686 Filler Metal Welds on CN3MN) ASTM G48a 1 cm
Temperature: 75° C 1cm
Solution: FeCl3 (Ferric Chloride) 1cm
1cm
6.2 wt% Mo
Weld Metal
Base Metal
7.9 wt% Mo 1cm
11.0 wt% Mo 14.0 wt% Mo
12.4 wt% Mo
Corrosion performance increases with decreasing dilution level
Corrosion Resistance as a Function of Dilution (IN72 Filler Metal Welds on CN3MN)
ASTM G48a 1 cm
Temperature: 75° C 1 cm
Solution: FeCl3 (Ferric Chloride)
3.2 wt% Mo 31.5 wt% Cr
5.1 wt% Mo 24.4 wt% Cr
Base Metal 1 cm
1.3 wt% Mo 38.5 wt% Cr
2.3 wt% Mo 34.9 wt% Cr
6.2 wt% Mo 20.6 wt% Cr
Weld Metal 1 cm
1cm
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
1 r o t a r e p O
Casting 3 2 r o t a r e p O
Inspection 1 1 r o t a r e p O
Foundry 3 2 r o t a r e p O
Inspection 2
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
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
All
Inspector 3
Box-and-Whisker Plot
Box-and-Whisker Plot
0.4
0
0.005 0.01 0.015 0.02
0.025
Standard Deviation
Standard Deviation
Standard Deviation
95% confidence interval for mean 0.0” to 0.04”
95% confidence interval for mean 0.0” to 0.2”
95% confidence interval for mean 0.0” to 0.02”
Punch Line – The resolution for all inspectors is about 0.4”.
0
0.1
0.2
0.3
0.4
Standard Deviation 95% confidence interval for mean 0.01” to 0.5”
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
Ny
=
G
& T
G: temperature gradient (K/mm)
Casting Simulation – Combine trial and simulation results: 6
Mean Minimum Niyama Values +/- One Standard Deviation
W/T = 2, CF-8M (20 Plates) • Trial results: X-ray level vs. FL W/T = 5.5, CF-8M (50 Plates) 5
W/T = 8, CF-8M (30 Plates) • Simulation of trials: Nymin vs. FL 5 W/T = 8, HH (20 Plates)
25
l e 4 v e L y a r X 3 e g a k n i r 2 h S M T S 1 A
6
0.005
4 X-ray level vs. Ny W/T = 8, (20 Plates) • Combine byHPeliminating → FL l e min W/T = 12, CF-8M (25 Plates)
6
Level 5: 25 Plates Level 4: 6 Plates Level 3: 12 Plates Level 2: 15 Plates Level 1: 24 Plates Level 0: 83 Plates
11
0.022
0.024 0.074
0 -1 0
0.1
TOTAL: 165 Plates
14
15
v e L 3 y a 2 r X 1
Nymin (K1/2s1/2mm-1)
9
0
13 70
0.2
Variation in the Ratings: Confidence intervals of x-ray level ratings grouped by average x-ray level 2.5
l a v r e t n I 2.0 e c n e d i f n o C %1.5 5 9 t t n e d u t S1.0 d e d i S e n O e 0.5 g a r e v A
0.0
Average one-sided student-t 95% confidence interval for all 128 x-rays = 1.42
Data from all foundries 27 x-rays 11 x-rays
35 x-rays
39 x-rays y a 5 r - . X 0 e < g l e a r v e e v L A
5 . 1 < g v a X < 5 . 0
8 x-rays
5 . 2 < g v a X < 5 . 1
5 . 3 < g v a X < 5 . 2
5 . 4 < g v a X < 5 . 3
8 x-rays 5 . 4 > g v a X
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 (%)
) % ( e m u l o V e r o P
macro-shrinkage
micro-shrinkage
0
log10(Ny )
Case Study #3: Valve B •
CG8M steel valve cast in a silica sand shell mold
•
After machining, 22% leaked around gasket on flange face during pressure testing
•
No shrinkage visible on x-rays
Casting defect. Leaks around gasket
outer diameter of flange = 5”
Case Study #3: Valve B
∼
•
leaking castings were sectioned, and photomicrographs were taken
•
microporosity evident in leaking area
0.1 mm
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
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 • 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)
leak
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
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
ID
Nymin = 0.69
OD
mid-plane mid-plane
Niyama Values in Region of Leak
ID
OD
photos and simulation results are from the valve mid-plane
Case Study #1 Summary • Simulation suggests potential for leak in valve – At valve mid-plane: Nymin = 0.58 (°C-s)1/2 /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
6
2
2
1 6
1 5
4
region of leak
4
5
Comparison: Region #1
Comparison: Region #2 A
B
A
B
A
B
Comparison: Region #3 B
A
B
A
B
Comparison: Region #4
Comparison: Region #5 A
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
OD
ID
Case Study #2 Simulations • Original rigging: OD
ID
• Nymin = 1.6 at OD, Nymin = 0.5 at ID
