Type Analysis | Description | Application | Corrosion Resistance | Scaling
Physical Properties | Workability | Typical Mechanical Properties | Applicable Specification

Type Analysis

Description

Carp 20 (20Cb-3) is an austenitic stainless steel possessing excellent resistance to hot sulfuric acid and many other aggressive environments which would readily attack Type 316 stainless. This alloy exhibits superior resistance to stress-corrosion cracking in boiling 20 to 40% sulfuric acid.Carp 20 stainless is also stabilized to limit intergranular attack, even in the sensitized condition. Corrosion tests on annealed

Application

This material has found wide use in all phases of the chemical and allied industries. It has been used extensively in the processing of syntheic rubber, high-octane gasoline, solvents, explosives, plastics, synthetic fibers, heavy chemical, organic chemicals, pharmaceuticals, and agrichemicals. 20Cb-3 (Carp 20) stainless has also been used in S0(2) scrubbers where acid content, such as sulfuric acid, is of more concern than high-halogen content, such as chlorides.
Other applications have included use in fans, mixing tanks, agitators, distillation towers, heat exchangers, process piping, bubble caps, metal cleaning and pickling tanks, spray pickling equipment, pump shafts and rods, valve stems, bolts, nuts, washers, tie rods, continuous-line pickling equipment including racks, etc.

Corrosion Resistance

The corrosion resistance of 20Cb-3 stainless in reagent-grade sulfuric acid should be used only as a guide, since many contaminants in commercial sulfuric acid can change the degree of corrosive attack. Corrosion rates in sulfuric acid are actually much lower in the presence of iron, copper, and chromium alloys, usually present in pickling and plating solutions. However, the presence of chloride ions can increase attack.

Stress -Corrosion Resistance
The higher nickel content of 20Cb-3 stainless confers even better resistance to chloride stress -corrosion cracking than that exhibited by 20Cb-3 stainless provided resistance to stress-corrosion cracking at a variety of temperatures and concentrations. It also displayed improved resistance to stress-corrosion cracking in boiling 42% magnesium chloride, a standard test for susceptibility to stress-corrosion cracking.

Sour Service
The excellent resistance of 20Cb-3 stainless to sulfide stress cracking has enabled inclusion of this alloy in NACE MR0175,"Sulfide Stress Cracking Resistance Metallic Materials for Oil Field Equipment." The alloy (UNS N08020) is acceptable in the annealed or cold worked condition at a hardness level of Rockwell C32 maximum.

Intergranular Corrosion Resistance
In the annealed and annealed plus sensitized (1250 °F, 1 hour) conditions, 20Cb-3 stainless will pass the Nitric Acid Test (240 hours in boiling 65% nitric acid) and the Ferric Sulfate-Sulfuric Acid Test (120 hours in a 50% sulfuric acid solution containing ferric sulfate) with a rate of 0.002 ipm maximum

Cleaning and Passivating
For optimum corrosion resistance, surfaces must be free of scale and foreign particles and finished parts should be passivated with diluted nitric acid.

Scaling

20Cb-3 stainless has a safe scaling temperature fo 1800 °F(982 °C) in continuous service

Physical Properties

Specific gravity ......................................8.08
Density
      lb/cu in ............................................0.292
      kg/cu m ............................................8080
Mean Coefficient fo thermal expansion

Thermal conductivity

Comparative type test conducted on a 2.5" square x 1.250" thick block derived from a 3-5/8" rod bar sample and reannealed 1725 °F/1 hour/W.Q.

Electrical resistivity (RT)                                                              Modulus of Rigidity
  ohm-cir mil/ft.............................................. 651                               psi x 10(6)............................................... 11.0
  microhom-mm............................................1082                               Mpa x 10(3)............................................ 75.8
Specific heat                                                                                  Poisson's Ration............................................0.31 
  Btu/lb-°F......................................................0.12                         Magnetic permability (H=200 Oe)
  J/kg-K............................................................500                        Annealed  condition................................... 1.002
Modulus of Elasticity (E) in tension                                              Cold reduced up to 80%............................ 1.002
  psi x 10(6)................................................... 28.0
  MPa.............................................................. 193

Workability

Hot Working
This metal can be forged, upset, headed and riveted satisfactorily. For forging, heat uniformly to 2100/2250 ° F. Do not forge below 1800 °F. After hot working operations, it should be reheated to 1725/1850 °F for a minimum of 30 minutes per inch of thickness and water quenched for optimum stabilization.

Stress Relieving
There have been occasions when stress relieving is desired for welded complex structures involving 20Cb-3 stainless. The following data, developed for specific case, indicate that stress relieving of 20Cb-3 stainless is safe at temperatures below 1000 °F(538 °C)

Effect of Stress Relieving on Corrosion Resistance of Welded Coupons in Ferric Sulfate-Sulfuric Acid - 20Cb-3 Stainless ASTM-A-262 Practice B - Rates shown in mils per year(mpy)

Machinability
20Cb-3 stainless, being an austenitic steel, machines with a tough chip but not quite as stringy as the 18-8's. It machines with a somewhat better finish than most austenitic steels. Suggested speeds for machining 20Cb-3 stainless are based on the use of standard high-speed tools . When using carbide tools, surface speed feet/minute can be increased between 2 and 3 times over the high speed suggestions. Feeds can be increased between 50 and 100%.

Welding
20Cb-3 stainless have been satisfactorily welded by the shielded fusion and resistance welding processes. Oxyacetylene welding is not recommended, since carbon pickup in the weld may occur. Since austenitic welds do not harden on air cooling, the welds should have good toughness. It is importance to keep the heat input and joint restraint to a minimum to avoid hot cracking. When a filler metal is required, AWS E/ER320LR welding consumables are suggested. These "low-residual" consumables greatly improve the resistance to weld hot cracking, which can be a problem, especially with higher heat inputs and thicker sections. The alloy has been used in the as-welded condition; however, for elevated temperature service, a postweld stabilizing heat treatment should be considered.

Typical Mechanical Properties

Typical Room Temperature Mechanical Properties
Annealed 1750 °F, water quenched.

Typical Elevated Temperature Tensile Properties

Applicable Specification

20Cb-3 stainless can be supplied to NACE MR0175; ASTM-A-625, B366, B462, B463, B464, B468, B471, B472, B473, B474, B475, B729 and their corresponding ASME specifications (as available).

Carp20 - Current Inventory Stock