Selecting Correct Valve/Pump Depends on Right Match of Alloy and Application

Pulp and paper mill engineers must be concerned with the corrosion resistance, abrasion resistance and quality of the cast stainless steels used for pumps and valves. These products obviously play a very important role in pulp and paper mills due to the tremendous amount of water, chemicals and other materials that a typical mill processes. However, much less information is published or readily available on the use of these cast alloys compared to the extensive literature on wrought alloys. This article provides guidelines for addressing these three cast stainless steel properties, and choosing the correct material for various mill applications.

Cast stainless steel compared to wrought materials. Data on the chemical composition of the principal cast stainless steels used for pumps and valves in the pulp and paper industry are shown in table 1. Information provided includes the cast designation, mechanical properties and pitting resistance equivalent number numbers (PREN). Alloys are grouped into the following types: martensitic, precipitation hardening, duplex, austenitic and super austenitic. Only the principal alloys in use are shown. Less widely used alloys of each type can be found in the applicable ASTM Specification. The side bar provides further details on the structure of the various alloys.

Cast stainless steels, like their wrought counterparts, have much improved quality when made using the argon-oxygen-decarbonization (AOD) process. Many, but not all, stainless steel foundries now use this process. Cast alloys have higher silicon content, which improves fluidity in pouring, and 5% to 10% ferrite, which reduces hot tears and micro fissuring. Their wrought counterparts by comparison have lower silicon and an austenitic structure with little or no ferrite. Despite these inherent differences, the corrosion resistance of AOD-produced stainless steel castings is quite comparable to the corrosion resistance of their wrought counterparts.

Corrosion Resistance in pulp mill and paper machine environments. Mill engineers are generally familiar with the corrosion behavior of austenitic stainless steels and the increasing resistance to corrosion that is provided by inclusion of molybdenum. Table 2 reports the resistance to crevice corrosion in fresh and brackish waters (at neutral pH and room temperature) of several of the alloys that have been extensively studied. The chloride content should always be considered in selecting alloys for pulp mill and paper machine environments.

In recent years a semi-quantitative method of estimating relative corrosion resistance in acid chloride environ- ments based on composition has been developed. It’s derived from the alloys' critical pitting temperature in ferric chloride. Data on the various alloys that are provided in Table 1 include the PREN or pitting resistance equivalent number for each.

Mill engineers can use PREN to estimate the corrosion behavior of a material relative to CF-3/CF-8 and CF-3M/CF-8M. Type CF-3/CF-8 without Mo has a PREN of 19. The martensitic and precipitation hardening alloys have a significantly lower PREN and have proven to be somewhat less resistant in service than CF-3/CF-8. CF-3M/CF-8M alloys, with 2% Mo, have a PREN of 25 which clearly identifies the much improved resistance of these products to pitting compared to Mo free CF-3/CF-8 grades. The CG-3M/CG-8M grades, with 3% Mo, have a PREN of 29 and somewhat better resistance than CF-3M/CF-8M. The duplex alloys have PRENs of 30-35 and thus may be used wherever the 3-4% Mo grades are applied. In many cases the duplex alloys are an upgrade. The super austenitic grades CK-3MCuN and CN-3MN with 6% Mo have PREN of 38 and 41 and have become the principal alloys used for bleach plant pumps in the C and D bleaching stage.

The PREN is particularly useful for evaluation of a proprietary cast alloy offered by a vendor in lieu of one of the alloys shown in Table 1. Mill engineers can calculate PREN for the offered alloy based on its composition using the PREN formula:

PREN = %Cr + 3.3x %Mo + 16x %N

The calculated PREN will allow the mill engineer to make a good first approximation of the corrosion behavior, comparing it to alloys he is familiar with.

Abrasion Resistance in the Pulp mill. Many mill process streams contain materials that are abrasive. The ability of a surface to “work harden” seems to be very important in determining erosion/ abrasion resistance of various materials. Austenitic alloys have good work hardening characteristics that seem to increase with alloy content. Erosion/abrasion resistance increases with hardness. Work hardening, hardness and tensile strength are roughly proportional for the austenitic and duplex alloys in Table 1. In other words, the alloy with the higher tensile strength will normally have the higher hardness and the better work hardening characteristics.

Table 3 is a rough guide for the relative erosion/abrasion resistance of various alloys. The alloys have been divided into six groups of roughly equivalent abrasion resistance. Materials in group I are used in the most severe service. The 28% Cr white iron has enough Cr to have better corrosion resistance than carbon steel, but it and the martensitic grades lack the good corrosion resistance of the austenitic steels. The 28% Cr alloy is used where maximum abrasion resistance is the principal consideration. Stellite 6, and other hard facing alloys, are a good second choice where abrasion is severe and abrasion resistance the principal consideration. The hard facing alloys are usually applied as a weld overlay on worn CF-3 and CF-3M pump cases.

Martensitic and precipitation hardening alloys (group II) are generally the first choice for the more severe abrasive services until experience shows that abrasion is extremely severe. In these cases only 28% Cr white iron or a Stellite 6 weld overlay will suffice.

The duplex alloys, group III, have been the materials of choice for pumps where sand and dirt carried through from the wood yard have eroded the softer, more common austenitic grades. This type of sand erosion/abrasion has become very prevalent, especially in pumps, throughout the front end of the mill. Therefore, the 3A and CD-4MCuN alloys rank second to CF-3 and CF-3M in pumps and may, in time, displace these grades.

Group IV materials seem to have slightly better resistance to mildly erosive/abrasive services, as compared to the less highly alloyed, more common materials in group V. Group V materials, although widely used, are easily eroded by sand, grit, chips, fibers and similar materials found in process streams.

Unlike corrosion, it is not possible to generalize an alloy's erosion/abrasion resistance with one test, as abrasion can vary widely under different test conditions such as load, speed, heat treatment, etc. Therefore, while simple selection factors like hardness or tensile strength can by used for ballpark rankings, specific site experience is the best and most reliable guide as to when an upgrade in abrasion resistance is required.

Suggested mill applications for various alloys. Table 4 identifies some of the principal applications for the alloys listed in Table 1. The CF-3 and CF-8 alloys are preferred for digester blow valves, compared to CF-3M and CF-8M. The Mo in the CF-3M and CF-8M grades adds little, if anything, to corrosion resistance in digester liquor service.

In the bleach plant, the two 6% Mo alloys have replaced CG-3M and CG-8M for the chlorine and chlorine dioxide stages. However, these 6% Mo alloys offer no advantage over the lower Mo content grades in oxygen, H2O2 or ozone bleaching service. CN-7M is not as widely used in the pulp and paper as in the chemical industry. It is included primarily because some manufacturers offer CN-7M valves as an alter- native to CF-3M/8M at virtually the same price in order to better manage their inventories.

Table 1: Typical Composition and Mechanical Properties of Cast Stainless Steels used in Pumps and Valves and PREN.

 
Procurement guidelines. Although mill engineers have full control of alloy selection, they have only an indirect influence, through the pump or valve manufacturer, on casting quality. The pump or valve manufacturer has more influence over casting quality through procurement practices, but it is the foundry that has direct control. However, there are several things mill engineers can do that will improve the quality of the cast pumps and valves they purchase. These steps add very little, if any, additional cost to the manufactured items. The following are the key procurement guidelines:

1. Insist on documentation.

2. Select and specify the low carbon grades of the common alloys.

3. Specify procurement to ASTM A744 when it is necessary to solu- tion anneal after all weld repairs have been made.

Documentation Details. Mill engineers should always ask for the following:

•Certification of chemical composition and mechanical properties

•Hydrostatic test certification

•Furnace temperature charts for solution annealing

•Foundry welder and welding procedure qualification reports

Chemical composition and mechanical properties are routinely supplied. Not all ASTM specifications require hydro testing, but nearly all manufacturers do.

Furnace temperature charts are not normally furnished unless specifically requested. It is highly desirable to have documentation on the solution annealing temperature for the molybdenum grades because many foundries do not have furnaces that will reach temperatures above 1900-1950F. Others have furnaces that will reach temperatures of 2000F, high enough to homogenize the molybdenum that gives the Mo grades their improved resistance to pitting. Knowing the annealing temperature the foundry used is essential, should unexpected corrosion occur in service.

Foundry and welder and welding procedure qualification reports are normally on file, but are not usually furnished unless specifically requested.

Documentation is a matter of good housekeeping and maintaining good records at very little, if any extra cost, if routinely requested during original procurement.

Table 2: Resistance to Crevice Corrosion in Fresh and Brackish Waters of pH 7-8.

 

Select and specify low carbon grades. It is standard practice for foundries to grind out and weld repair visible defects in large castings such as those for pulp and paper pumps and valves. For this reason, mill engineers should specify the low carbon grades such as CF-3, CF-3M and CG-3M. The higher carbon grades, 0.08% C max, will be sensitized to inter-granular corrosion by these weld repairs, normally without advising customers their castings have been weld repaired. Their low carbon content does not eliminate the need to solution anneal as some suppliers have contended.

Specify ASTM A744 when needed. In certain environments, it is necessary to insure that the castings have been solution annealed after all weld repairs have been made. When mill engineers have been advised or have learned that this is necessary for a particular service, the pump or valve manufacturer can be required to procure castings to ASTM A744. This requires solution annealing after all weld repairs, rather than ASTM A743, which permits weld repairs after solution annealing, and is normally used. An extra cost will be incurred for procurement to A744 so procurement to A744 should be specified only when experience or corrosion specialist's advice indicates the extra cost is justified for the intended service.

Table 3: Relative Erosion/Abrasion Resistance.

1. Ferrite Content of Cast Austenitic Grades. A little ferrite is good. Too much is bad. The cast austenitic grades are designed to have 5-15% ferrite which improves casting and welding. Nadezhdin2 et al. have drawn attention to weeping, corrosion and cracking failures of CF-3M pumps with 25% ferrite and high porosity. Forty-four cast stainless steel pump casings and other cast components from three different suppliers were analyzed for ferrite content. Ferrite ranged from 10.4% to 24%. The most serious failures were associated with the higher ferrite contents.

Ferrite content is not controlled by the ASTM specifications and is only rarely controlled in procurement. Most foundries hold ferrite content below 18% but some do not. Methods for measuring ferrite content are set forth in ASTM A800. When unexpected corrosion or cracking problems arise, it is good practice to check the ferrite content.

2. Duplex Grades. ASTM A890 3A, CD-6MN, cast duplex stainless steel has been used for many years for pumps in the pulp and paper industry. It is probably second in terms of tonnage of cast alloys used for pumps to CF-3M. Two grades of CD-4MCU are covered in A890; Grade 1A without nitrogen, and Grade 1B with nitrogen. Grade 1B should always be specified because the nitrogen addition is critical to best performance of this grade. The three most widely used duplex grades, 3A, CD-4MCuN, and J92205, have substantially higher PRENs than CG-3M/CG-8M and may be used in place of, or as an upgrade, in services where CG-3M/CG-8M has been employed.

3. Super Austenitic 6% Molybdenum Grades. The CK-3MCuN and CN3MN grades are two of the newer 6% Mo cast "super austenitic" stainless steels. They are the cast counterparts of S31254 and N08637 respectively, which have become standard for washers, vats and piping in the more aggressive bleach plant environments.

Super austenitic stainless steels in cast form have been used as chlorine and chlorine dioxide bleaching stage pumps and valves with good success over the past six years.

Full documentation, especially the furnace charts, are important for these grades. Mills can specify either 6% Mo grade under ASTM A743 or A744. Although more expensive, procurement to A744 is recommended, which requires solution annealing after all foundry weld repairs. A caution to mill engineers is warranted here. Mill engineers should definitely reject/ban foundry requests to solution anneal at temperatures below 2100F for longer times, as some foundries (with furnaces not designed for the high temperatures required) frequently offer to do. Heating for a longer time at a lower temperature destroys much of the excellent corrosion resistance of these highly alloyed materials and should never be considered acceptable.

ALLOY 654SMO: This is an even newer and more corrosion resistant high Mo, high N, wrought composition that has been cast successfully for three large valves for D stage service. There is no cast designation as yet and no ASTM specification. It is included as a matter of interest.

Table 4: Common Usage of Cast Stainless Steels.

 

4. Major/minor welded repairs. ASTM Specification A703, "Standard specification for steel castings, general requirements, for pressure containing parts", defines major repairs. Major repairs are those that either result in: 1) leakage on hydrostatic test; 2) are > 20% of wall thickness; 3) are > 1" (25mm) in depth; or 4) exceed 10 sq in. (65 sq cm). All other weld repairs are considered minor. Most specifications permit repair of minor defects without notification to the customer.

ASTM A890, to which the duplex grades are normally procured, does not require post-weld repair heat treatment unless so stipulated by the purchaser.

ASTM A747 controls the conditions for weld repair and reheat treatment of the precipitation hardening grades.

5. Other inspection and casting quality specifications and standards. Visual Examination: ASTM A802 and Manufacturers Standardization Society of the Valve and Fitting Industry, (MSS) SP-55, provide criteria for visual inspection of castings. A802 bases inspection on Steel Casting Research and Trade Association (SCRATA) graded reference comparisons. MSS SP-55 uses photographic standards but includes a table that correlates these to those of A802.

MSS SP-93 Liquid Penetrant (LP), MSS SP-94 Ultrasonic (UT) and MSS SP-54 Radiographic Examination standards provide useful criteria for each NDT method. LP will disclose hairline cracks and other surface defects, but not subsurface defects. UT will detect subsurface defects on machined flat surfaces, and with less accuracy, on smooth curved as cast surfaces. Radiographic examination is the most powerful and most expensive method of insuring integrity and high quality in castings. Radiographic examination can usually be limited to the obviously critical sections of pump and valve castings which normally mean that no more than 10% of the total area needs to be examined.

Alloy Applications for non-chlorine Bleach Plants. Oxygen, ozone and hydrogen peroxide are not as aggressive as chlorine and chlorine dioxide. Therefore these chemical environments do not require the most corrosion resistant alloys. CG-3M/CG-8M is expected to predominate for these services, except when sand loadings are a factor. When sand is a factor, the duplex compositions are preferred.

Conclusions. 1. Mill engineers control selection of the alloy to be used. Specifying low carbon grade austenitic alloys and nitrogen-containing duplex alloy grades can avoid many problems.

2. Mills do not have direct control over the quality of the cast stainless steels used in pumps and valves. Suggestions mill engineers should consider for inclusion in all procurement for pumps and valves include documentation, specifying the low carbon grades and, when appropriate, specifying procurement to ASTM A744 instead of A743.

3. Mill engineers can use the PREN number of a known or newer alloy as a guide to its corrosion resistance in most pulp and paper mill applications.

4. Alloy selection for wear and abrasion resistance is usually site and application specific because there are so many influencing variables. Some general guidelines are given.

5. The newer cast 6% Mo austenitic stainless steels are performing well in aggressive bleach plant services.

6. Introduction of oxygen, ozone and hydroxide bleaching processes pose no significant problem for the CG-3M/CG-8M and duplex alloys now being used for pumps, valves, and other cast components. The more highly alloyed super austenitic alloys are unlikely to be required for these services. *

REFERENCES

1. Tuthill, A. H., "Procurement of Quality Stainless Steel Castings," TAPPI Journal, September 1998.

2. Nadezhdin, A., Cooper, K., and Timbers, G., "The Ferrite Content and its Effect on Corrosion Resistance of Stainless Steel Pumps in Bleach Plant Services," NACE Corrosion/94, Paper Number 430.

ACKNOWLEDGEMENTS

The authors wish to express their appreciation to the Nickel Development Institute for their support in the preparation of this paper.

Arthur Tuthill is president of Tuthill Assoc. Inc., Blacksburg, Va., and Alan P. Castillo is a former consultant of C.C. Technologies Inc., Dublin, Ohio.