NHML Resources - Paper Rolls

The conditions we see that call for the repair/rebuilding of large paper making rolls are either 1) that a hard surface has worn and been reground repeatedly until grinding has broken through to the softer core, or 2) with repeated regrinding the locked up stresses have become unbalanced and the roll or its journals are no longer straight.

White Iron Rolls

White cast iron is hard and can be formed in thick enough layers so rolls can be reground repeatedly. Cast irons that have low side carbon and low side silicon will form gray iron when they are slow cooled while they tend to form white iron when they are rapidly cooled. The commonly encountered elements that increase the depth of the white iron include chromium, vanadium, molybdenum, boron, and manganese. Those that decrease the depth are aluminum, titanium, cobalt, copper, nickel, and phosphorous. Carbon and silicon are tricky. More carbon increases the hardness but also cuts down on the depth of the case, as well as interfering with forming white iron. Silicon is required for making cast irons but decreases the case depth.

In white iron, it is not the chemistry that is important to the end user; it is the hardness and depth that are important. Also, since most rolls are either heated or cooled, it is important that they be pressure tight.

Centrifugal Cast White Iron/Gray Iron - Dual Melt

A melt having a composition that will form white iron is poured into the centrifugal mold while it is rotating. After the white iron shell has frozen, a melt having a composition that will form gray iron is poured in. This technology is covered by ASTM A942.

The hardness of the white iron may be specified.

ASTM A942 Type and Hardness
Type I 450-500 HB
Type II 500-550 HB
Type III 550-600 HB
Type IV 600-650 HB

ASTM E140 Approx. Conversion
Type I 45-50 HRC
Type II 50-53 HRC
Type III 53-56 HRC
Type IV 56-58.5 HRC

The core can be specified to any of several of the lower strength gray cast irons.

If you are ordering rolls made by this method, note that the mold has "crop ends" for both the white iron and the gray iron, and that the hardness, chemical analysis, and tensile testing are done on these ends.

Statically Cast White Iron/Gray Iron

In this technology a gray iron core is placed in the mold and a white iron shell having a different composition is cast around it. Both the mold and the cast iron core chill the case. The technology is covered by ASTM A748. The hardness requirement for the case is minimum 60 Scleroscope "C". This does not convert very well into other hardness tests, however, it is in the vicinity of HRC 46. The core can be gray cast iron grades 20 through 35. (ASTM A278). Core tensile bars are taken from extended ends on the journals.

The case depth requirement is max. 30% of the total wall thickness. It is measured on the end of the roll.

Centrifugal Cast White Iron/Gray Iron - Single Melt

A single melt is cast against an external chill which results in the rapid cooling on the outside that creates the white iron shell. Under the white iron there is a transition zone called mottled iron, and below that is the gray iron core. The case hardness is minimum 55 Scleroscope "C" which converts to approximately HRC 42.5. The core is a Grade 20 gray cast iron. The case thickness is between 5% and 30% of the finished wall thickness, determined by ultrasonic testing. We have seen these cases over 2 inches thick, allowing for numerous regrinds. Be sure that the ultrasonic test instrument is calibrated against a white iron standard. White iron is so metallurgically different from steel and gray iron that it needs its own calibration blocks.

When to Replace / Rebuild

Between the gray iron core and the white iron case of the single melt rolls, there is an area called "mottle" which is mixed white and gray iron. Rolls made by one of the dual melt processes have very thin transition areas so that there may be little or no mottle.

With single melt rolls, either in service or during regrind, splotchy areas may appear that are darker than the rest of the surface. These may be mottle, which indicates that there can be no further grinding. After the roll has cooled to room temperature, the metallurgist will use portable polishing equipment on a gray splotch and on a white area. The areas will be etched and examined using a portable metallurgical microscope. The metallurgist will report whether the observed microstructures are white, white with graphite flakes, mottle, or gray. If appropriate, hardness tests will be made.

With dual melt rolls, when wear or grinding has broken through, instead of seeing splotches we have seen large areas that appear darker as compared to the white iron. The metallurgist uses the same method to confirm the microstructures.

Rebuilding rolls by the ASTM A748 method of pouring white iron around the core is possible. The journals usually need to be built up. To prevent distortion in service, after rough machining the whole assembly needs to have a very lengthy stress relief heat treatment.

Steel Rolls

The rebuilding method of choice is submerged arc welding. Timken Co. of Canton, Ohio markets filler metal wire similar to stainless steels 420 and 423, a higher chrome version, and an alloy similar to H11. These require heat treating to develop their strength and hardness potential. After heat treatment, hardness can be in the HRC 32-55 range. For a discussion of rebuilding hot working rolls, see Advanced Materials&Processes, March 2005, p. 43, from which Fig. 1 was taken. Providing there is adequate stress relief, the method works well for rolls used in the paper industry.

Purchase Specifications

When specifying new rolls, we strongly recommend invoking the ASTM specifications listed above. Purchase your copies online from ASTM. In your PO, reference each of the options that you choose to invoke. It is never sufficient to simply list the ASTM specification number. Failure to properly reflect the options offered by the spec will result in your not being able to reject a bad roll. Also be sure to resolve all conflicts between your print and the ASTM spec; failure to do so will result in disappointment.

In addition to the specifications listed above, your roll supplier is probably prepared to work to ASTM A532, Standard Specification for Abrasion-Resistant Cast Irons. This specification offers options in chemistry and hardness. In chemistry there are Classes I, II, and III, and within each class there are one to four optional Types. These Classes and Types carry specific hardness minimums.

In writing your PO for a new or rebuilt roll, please check that your hardness requirements are in the same scale as the ASTM specification. If your QC department isn't prepared to test to the scale called out by the chosen ASTM spec, then write into the PO what scale and numerical values are acceptable. In drawing up an alternate hardness scale and values, you can select values from ASTM E140 Table 8 which gives approximate hardness conversion numbers for the alloyed white irons. These white cast hardness conversion numbers are somewhat different from the conversions in the tables typically supplied by hardness test instrument makers, which are usually E140 Table I covering non-austenitic steels. Table 8 does not include Scleroscope hardness conversions. We have seen an instance in which failure to write into the PO the acceptable hardness scale and values resulted in a court backing up the roll supplier in denying a claim based on inadequate hardness.

NHML Services

Either in your mill or at a rebuilding shop we will determine microstructures and hardness. Hardness testing in the field includes Rockwell B, C, and Scleroscope (other scales are available for testing on non-ferrous piece-parts). Chips for chemical analysis can be taken from the white layer, core, and coatings.

Rebuilding shops working on steel rolls usually need to know the core composition, and there is always the possibility that journals are different. We analyze metals by ICP-AE (Inductively Coupled Plasma-Atomic Emission) using NIST standards with carbon and sulfur determined on a dedicated spectrometer using NIST traceable standards. Plastics, greases, and organic coatings are analyzed by multiple techniques that include infrared spectroscopy and GCMS (Gas Chromatography Mass Spectrometry). If a roll is still in the machine, it will need to be cooled to less than about 90°F before we examine the microstructures; experience has shown that the etchants do not perform properly at elevated temperatures.

Our on-site work can often be more productive if the chemical analysis work, if any, has been completed in advance. Call and we will give you instructions on how to collect the samples and which contamination issues may be of concern. While we are on site, feel free to bring to our attention other metallurgical and materials issues.

Depending on local circumstances, some of our on-site work is performed through Consolidated Consultants of Yarmouth, ME.

See our Industry Definitions for further insight.

 

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