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Revealing PAGBs
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Transcript of Revealing PAGBs
Revealing Prior-Austenite
Grain Boundaries
George F. Vander Voort
Consultant, Struers Inc.
March 17, 2010 2
Examples of Phase Decoration
Depending upon the steel composition, a variety of
procedures can be used to decorate the prior-austenite
grain boundaries – if the steel can be heat treated.
Examples include ferrite or cementite decorating the
grain boundaries after slow cooling or after carburizing,
or using the oxidation method.
If the specimen can not be heat treated, as in failure
studies, we can only use etchants to bring out the prior-
austenite grain boundaries.
March 17, 2010 3
Examples of Grain Boundary
Decoration by a Second Phase
Ferrite precipitated in the prior-austenite grain boundaries in an as-cast Fe-0.38% C –0.26% Si – 0.79% Mn steel (2% nital).
March 17, 2010 4
Examples of Grain Boundary
Decoration by a Second Phase
Ferrite decorating prior-austenite grain boundaries in a wrought medium carbon steel, 2% nital
March 17, 2010 5
Examples of Grain Boundary
Decoration by a Second Phase
Fe – 1.31% C steel, as-rolled, cementite outlining PAGBs: 2% Nital (left), 4% picral (right). Neither clearly reveals the grain boundaries.
March 17, 2010 6
Examples of Grain Boundary
Decoration by a Second Phase
Fe – 1.31% C as-rolled: Alkaline sodium picrate, 90C, 60 s, 500X (left); Klemm’s I (right); both revealed the grain boundaries.
March 17, 2010 7
Examples of Grain Boundary
Decoration by a Second Phase
Microstructure of 9310 alloy steel after the McQuaid-Ehn test using alkaline sodium picrate
(90 C – 45 s) to darken the cementite precipitated in the prior-austenite grain boundaries
(magnification bar is 100 µm long).
March 17, 2010 8
Examples of Grain Boundary
Decoration by a Second Phase
In high-alloy steels it is often possible to precipitate a second phase, such as pearlite, on the prior-austenite grain boundaries so that they can be easily examined. This experimental 5% Cr hot-work die steel specimen was austenitized at 1080 °C, cooled to 700 °C and held for 1
h, then air cooled to precipitate pearlite on the grain boundaries. It was etched with glyceregia. Magnification bar length is 25 µm.
March 17, 2010 9
Oxidation Technique
The specimen was ground to a 400-grit SiC finish, then austenitized, quenched, and lightly reground but not polished.
March 17, 2010 10
The Most Successful Etchant
1. Saturate water with picric acid
2. Add 1 mL HCl to 500 mL of the saturated solution
3. Add 1 large tablespoon of a wetting agent (I use
Nacconal 90G, Stepan Co., Northfield, IL
Make 500 mL of the following solution using a magnetic
stirring plate and an 800 – 1000 mL beaker
Dissolve thoroughly. Filter at least 100 mL into a 240 mL
beaker. Heat solution to 80-90 C. Swab or immerse specimen
for 40-60 s. Wash and dry. Examine with LOM. Light back-
polishing with 0.05 µm alumina slurry on a stationary cloth to
remove excess matrix etching. Etch again and back-polish
until the boundaries are clearly revealed.
March 17, 2010 11
Etching Examples
Back-Polished Once Back-Polished Twice
Prior-austenite grain boundary delineation improved by back-polishing after
etching to remove excessive etch attack in A-350 (LF3) alloy steel.
March 17, 2010 12
Etching Examples
Specimen B1, mount T3, 2% nital (left, 50X); saturated picric at 20 C (right, 200X)
Etching with nital reveals packets of lath martensite; the aqueous saturated picric acid solution with HCl and a wetting agent at 20 C faintly revealed the prior-austenite grain boundaries in SAE 723, Grade 3, Class 3 pressure vessel steel (Fe - 0.33% C – 0.25% Mn – 0.13% Si – 3.55% Ni – 1.66% Cr – 0.48% Mo – 0.12% V )
March 17, 2010 13
Etching Examples
Specimen B1, Mount T3, saturated picric at 90 C (left, BF, 200x); DF (right, 200X)
Using aqueous saturated picric acid plus HCl and a wetting agent at 90 C, however, revealed the
prior-austenite grain boundaries (left: bright field illumination; right: dark field illumination); in SAE 723, Grade 3, Class 3 pressure vessel steel .
March 17, 2010 14
Etching Examples
Specimen B1, mount L1, 2% nital (left, 50X); saturated picric at 90C (right, 200X)
Etching with nital reveals packets of lath martensite; the aqueous saturated picric acid solution with HCl and a wetting agent at 90 C revealed the prior-austenite grain boundaries in SAE 723, Grade 3, Class 3 pressure
vessel steel (Fe - 0.33% C – 0.25% Mn – 0.13% Si – 3.55% Ni – 1.66% Cr – 0.48% Mo – 0.12% V )
March 17, 2010 15
Etching Examples
Specimen B1, Mount L1, saturated picric at 90 C viewed with DF at 200X)
Using aqueous saturated picric acid plus HCl and a wetting agent at 90 C revealed the prior-
austenite grain boundaries as shown in dark field illumination); in SAE 723, Grade 3, Class 3 pressure vessel steel .
March 17, 2010 16
Etching Examples
2% Nital Sat. Pic, 90C – 2 min
Prior-austenite grain boundaries revealed in martensitic A-350 (LF3) alloy steel (Fe –
0.07% C – 0.74% Mn – 3.66% Ni – 0.2% Cr – 0.07% Mo (1350 F temper) using aqueous
saturated picric acid plus HCl and a wetting agent.
March 17, 2010 17
Etching Examples
As Quenched 400 ºF Temper
Prior-austenite grain boundaries revealed in 8620 alloy steel using aqueous
saturated picric acid plus HCl and a wetting agent at 80-90 ºC, 60s.
March 17, 2010 18
Etching Examples
800 F Temper 1200 ºF Temper
Prior-austenite grain boundaries revealed in 8620 alloy steel using aqueous
saturated picric acid plus HCl and a wetting agent at 80-90 ºC, 60s.
March 17, 2010 19
Etching Examples
400 F TemperAs-Quenched
Prior-austenite grain boundaries revealed in fully martensitic 4140 alloy steel using aqueous saturated picric acid plus HCl and a wetting agent at 80-90 C.
March 17, 2010 20
Etching Examples
1200 F Temper800 F Temper
Prior-austenite grain boundaries revealed in fully martensitic 4140 alloy steel using aqueous saturated picric acid plus HCl and a wetting agent at 80-90 C.
March 17, 2010 21
Etching Examples
41B50, 1650 F – 550 F Isothermal Hold 5160, 1600 F – 600 F Isothermal Hold
Prior-austenite grain boundaries revealed in two alloy steels isothermally
transformed to lower bainite using aqueous saturated picric acid plus HCl and a
wetting agent.
March 17, 2010 22
Etching Examples
300 F Temper 500 F Temper
Prior-austenite grain boundaries in 4340 alloy steel isothermally transformed to lower bainite using aqueous saturated picric acid plus HCl and a wetting agent.
March 17, 2010 23
Etching Examples
700 F Temper 900 F Temper
Prior-austenite grain boundaries in 4340 alloy steel isothermally transformed to lower bainite using aqueous saturated picric acid plus HCl and a wetting agent.
March 17, 2010 24
Etching Examples
300 F Temper 500 F Temper
Prior-austenite grain boundaries revealed in fully martensitic 4340 alloy steel using aqueous saturated picric acid plus HCl and a wetting agent.
March 17, 2010 25
Etching Examples
Stress-corrosion cracks (SCC) in 4340 alloy steel nut risers (39 HRC) etched with saturated aqueous picric acid, 80 C, 500X (magnification bars are 20 um long).
March 17, 2010 26
Etching Examples
Prior-austenite grain boundaries in 43B17 alloy steel (Fe – 0.17% C – 1.85% Ni – 0.75% Cr – 0.25% Mo –0.003% B) revealed by etching with aqueous saturated picric acid plus 1% HCl and a wetting agent. The
specimen at the left was given a normal heat treatment (843 °C – 30 min, oil quench, 230 °C temper) and has a fine grain size. The specimen on the right was “mock carburized” (925 °C for 8 h, oil quench and tempered at 230 °C) and rapid grain growth has begun. This specimen is not suitable for carburizing.
March 17, 2010 27
Etching Examples
Prior-austenite grain boundaries revealed in fully martensitic Modified 4330V alloy steel (Fe –
0.29% C – 0.39% Mn – 3.54% Ni – 1.69% Cr -0.54% Mo – 0.11% V (1110 F temper) using
aqueous saturated picric acid plus HCl and a wetting agent.
March 17, 2010 28
Etching Examples
Prior-austenite grain boundaries revealed in fully martensitic 5160 alloy steel using aqueous
saturated picric acid plus HCl and a wetting agent.
5160, 1525 F – OQ – 300 F Temper
March 17, 2010 29
Alternative Etchants
March 17, 2010 30
Etching Examples
Solution Annealed – 60% HNO3 – 1.1Vdc Solution Annealed and Aged - Superpicral
Prior-austenite grain boundaries can be revealed easily in precipitation hardenable stainless steels and in maraging steels if the are in the solution annealed condition (before aging). This
specimen of 17-4PH stainless steel (Fe - <0.07% C – 17% Cr – 4% Ni – 4% Cu – 0.3% Nb) was solution annealed at 1038 °C. It was etched with aqueous 60% HNO3, 1.1 V dc, Pt cathode, 30 s.
If they have been aged, it is more difficult but superpicral has been found to work.
March 17, 2010 31
Prior-austenite grain boundaries
18Ni250 solution annealed and aged maraging steel. Modified Fry’s reagent almost brings out the prior-austenite grain boundaries (magnification bar is
25 µm long).
March 17, 2010 32
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Examples of Grain Boundary
Decoration by a Second Phase