New materials drive development of thermo rolls for high speed calenders: traditional materials are not always sufficient for thermo rolls in modern calendering systems.

Paper companies are using more online, multi-nip calenders to produce high-quality paper grades. Paper machines up to 11 meters wide with production speeds up to 2000 m/min are now a reality. These operating conditions demand higher standards for the thermo rolls used in calendering systems. Operating criteria--such as reliability, process conditions, efficiency, and flexibility--are critical. A thermo roll used in a high-speed modern calender, for example, requires the best possible materials and an optimized design for specific operating conditions (Figs.1 and 2).

[FIGURES 1&2 OMITTED]

ROLL MATERIALS

Thermo rolls for most calender applications have traditionally been manufactured from chilled cast iron due to its wear resistance. However, modern calendering systems require a different approach. For example, thermo rolls for modern calenders must provide heating capacities of up to 150 kW/m. Even optimized chilled cast iron grades used for high-temperature applications (such as the KT material from Walzen Irle) reach their operational limitations when used in modern high-speed calenders. Roll material selection depends on applied loads and the fatigue endurance limit of the material. All major loads applied to a thermo roll can be described in a "Haigh" diagram, which is a load graph showing the stress situation (Fig. 3).

[FIGURE 3 OMITTED]

Technicians enter the superimposed stress onto the abscissa, which is determined by the residual and thermal stress as well as the alternating bending stress onto the ordinate. The permissible operating window (shown in blue) for both the chilled cast iron materials (K and KT) as well as a specially developed chromium steel material (such as KSTV from Walzen Irle) is shown.

A safety factor of S > 2.0 is standard for all roll materials. Thermal stress is the most decisive factor in roll sizing for calenders with a heavy load. For example, a theoretical roll designed to have stresses located at point "A" of the diagram must be made of the higher strength KSTV material or from forged steel to stay within the safety factor. High-speed on-line calenders must be carefully designed for safe operations since failure of any roll may cause extensive damage.

ALTERNATIVE MATERIALS

Where chilled cast iron is not sufficient, alternative materials with better technological properties are available. Modified nodular iron materials with a pearlitic or acicular matrix (such as Walzen Irle KST-P and KST-A) are available. While these roll materials have better strength properties than standard chilled cast iron, they have drawbacks. Depending on the surface hardness, varying amounts of graphite can form on the roll surface, reducing wear resistance. This can be minimized by surface coating the roll in high wear applications.

Other materials--such as KSTV, a homogeneous material with steel-like properties--can supply wear resistance without the need for surface coating. Chemical composition varies for different applications based on the strength, surface hardness, and wear resistance needed. Figure 4 shows a comparison in the microstructure of KSTV versus chilled cast iron KT.

KSTV has a hardened matrix where, depending on the application, different amounts of Cr-carbides can be embedded. The Cr-carbides are finely striated, having a hardness of approximately 1800 HV and would create, together with the hardened matrix, an improved wear resistance material. A surface hardness of up to 700 HV (86 Sh C) is obtainable. KSTV offers longer grinding intervals compared to a conventional chilled cast iron roll. The hardness of a KSTV-roll is almost constant over the entire wall thickness, so that after longer periods of use there are no changes in the wear behavior (Fig. 5).

[FIGURE 5 OMITTED]

Over 30 installations with KSTV have demonstrated the material's superiority to conventional chilled cast iron. Table 1 summarizes the essential mechanical properties of materials K, KT, KST-P, KST-A and KSTV. Achievable Surface hardness values, tensile strength properties and the modulus of elasticity of KSTV is far above those of other materials. This positively influences the 1/2 critical speed of the thermo roll and the sizing of the deflection compensating roll. In many cases, mills can use a smaller diameter KSTV-roll without reaching the strength limits of the roll. The smaller diameter produces a higher specified nip pressure, which has a positive influence on paper smoothness--a major benefit in roll weight- or size-limited applications.

FORGED STEEL OPTION

For applications that exceed the capabilities of KSTV material, forged steel is an option. Aside from ideal heat conductivity, the forged steel also has excellent strength and shape stability. However, one disadvantage is its insufficient basic hardness/wear resistance. It is definitely better to cover this type of roll with a wear resistant surface coating. Although it is possible to harden the surface, we do not recommend this approach to achieving wear resistance due to the material stress at the transition layer.

Figure 6 depicts the microstructure of KSTV material compared to a hardened forged steel. The wear resistance of the forged steel is relatively poor since it contains no carbides. The hardness of the forged steel depends entirely on the hardness of the matrix. Limited experience has shown that forged steel also provides less damping characteristics compared to chilled cast iron.

[FIGURE 6 OMITTED]

Several years ago, S.D. Warren Company (now SAPPI Fine Papers North America) developed with the assistance of Walzen Irle heated calender rolls using forged steel with a spray coated surface to provide superior wear properties. This patented technology is applicable to modern high-speed calenders. To date, Walzen Irle has delivered 11 forged steel rolls to SAPPI North America. Since SAPPI holds the patent rights for coated thermo rolls made of forged steel for the paper industry, Walzen Irle and S.D. Warren have entered into an agreement that allows Walzen Irle to manufacture and sell the forged steel rolls under an exclusive license.

Table 2 summarizes and compares the important criteria for three roll materials: chilled cast iron, KSTV, and forged steel. Operating reliability, running behavior and wear resistance all favor the KSTV and/or forged steel materials.

ROLL OF THE FUTURE

Thermo rolls made of a homogeneous material such as KSTV or forged steel with a wear resistant coating will continue to replace conventional rolls. These tolls give papermakers optimal wear resistance and tensile strength properties, producing a truly stable operating roll for demanding high-speed modern calenders.

Material Properties

                                        Chilled Cast Iron
Criteria                                        K
                                         white       grey

Hardness              [HV]             500-600     220-300
Tensile Strength      [N/[mm.sup.2]    200-250     150-220
Bending Strength      [N/[mm.sup.2]    360-450     260-400
Alternating Bending   [N/[mm.sup.2]     75-80       70-80
Strengths
Modul of Elasticity   [kN/[mm.sup.2]   170-185     95-115

                      Chilled Cast Iron            Modular Iron
Criteria                       KT
                      white             grey          KST-P

Hardness              480-550          220-300      520 max. *
Tensile Strength      220-270          160-240       350-450
Bending Strength      400-480          290-430       570-730
Alternating Bending    75-80            80-90        140-180
Strengths
Modul of Elasticity   170-185          100-120       160-180

                                       Chromium
                                         Steel

Criteria                KST-A            KSTV

                      560 max. *       450-700
Hardness               450-550         600-650
Tensile Strength       730-900         900-1020
Bending Strength       180-220         200-210
Alternating Bending
Strengths              160-180         200-210
Modul of Elasticity

* Depending on roll dimensions

Criteria
                                         Chilled   KSTV   Forged Steel
                                        Cast Iron           hardened

Sale Operating, Tensile Strength            0       ++        +++
Heating capacity                            -       ++        +++
Residual Stresses                          ++       ++         +
Roll Sizing: Roll [empty set]               0       ++        ++
Modul of Elasticity; 1/2 crit. Speed        0       ++        ++
Shape stability under temperature           0       ++        ++
Running behavior, Balancing precision       0       ++        ++
Wear resistance                            ++       +++       + *
Flexibility [DELTA] T/min.                  +       ++        +++
Damping features                           ++        +         +

- = limited, 0 = neutral; + = acceptable; ++ = good; +++ = best

* depending on hardness

Tables 1 (top) and 2 (bottom) summarize
mechanical properties and criteria for roll
materials.

Wolfgang Wiertz is sales director for Walzen Irle GmbH, Huettenweg 5, 57250 Netphen-Deuz, Germany. Contact him at: wiertz@ walzenirle.com