Overall, the results of the exposure test shows that the bright- annealed and surface-finished specimens exhibited considerably higher corrosion resistance in this environment than the open-annealed and pickled counterparts. Sandvik has applied this knowledge to develop steels that can outperform insufficient standard grades, like 316L, in hydraulic systems. It is worth examining the metallurgical properties of these grades in more detail to better understand the advantages of bright-annealed and surface-finished tube and also to determine why standard grades are underperforming in hydraulic and instrumentation tubing applications.

Two useful examples are Sandvik 3R60, an austenitic chromium-nickel steel, and Sandvik SAF 2507TM (UNS S32750), a high-alloy and high-strength superduplex tube. Both come in a solution-annealed condition, either white pickled or bright annealed, and each has been shown to outperform standard materials, both in laboratory examinations and in field tests. Each was developed to exhibit the high mechanical strength required for hydraulic cylinders in aggressive environments, such as warm chlorinated seawater and acidic, chloride containing media. The anticorrosion properties of steels are integral to their mechanical strength; chromium content is particularly important for pitting resistance, and alloying with molybdenum and nickel has also proven beneficial.

Figure 4 compares the key alloy content of Sandvik 3R60 alongside levels typically found in standard 316/316L steels. Alloy levels in standard grades are often kept to a minimum in conformance to ASTM standards. As shown, Sandvik 3R60 has higher levels of Cr, Ni, and Mo than equivalent standard grades. In combination, these steels give the material a higher Pitting Resistance Equivalent (PRE) value than of typical steels. Sandvik SAF 2507 has a nominal PRE value of 43 (minimum 42), significantly higher than the PRE values for other duplex 25Cr stainless steels that are not superduplex. This is partially due to the fact that the material contains 4.0% Mo, giving it performance comparable to 6.0% Mo austenitic steels.

These characteristics have been put to the test with ASTM G48, one of the most-severe pitting and crevice-corrosion tests applied to stainless steels. The modified ASTM G48 test exposed Sandvik SAF 2507 to 6% FeCl3, with and without crevices, for periods of 24 hr. When pits were detected together with a substantial weight loss (>5 mg), the test was interrupted; otherwise the temperature was increased by 5°C, and the test continued with the same sample.

The scatter band in Figure 5 shows that Sandvik SAF 2507 and 6Mo+N alloys have similar resistance to pitting, and critical pitting and crevice temperature (CPT and CCT) values are within the range shown. Yet, Sandvik SAF 2507 has distinct advantages over these 6.0% Mo steels. For one, it is more readily available, giving it lower initial costs.

Tests of Sandvik 3R60 reveal that low-carbon content imparts higher resistance to intergranular corrosion than that exhibited by type ASTM 316 steels. Elsewhere, Sandvik 3R60 was exposed to corrosion testing for 24 hr in boiling Strauss solution (12% H2SO4, 6% Cu2SO4). Figure 6 shows results in a TTC-diagram and indicates that the material’s resistance to grain boundary attack is much better than ASTM 316L — an advantage in complicated welding operations.

As with Sandvik 2507, the superior resistance of Sandvik 3R60 to these types of corrosion is partly due to its 2.6% Mo content. The 3R60 material has substantially higher resistance to attack than steels of ASTM 304 steels, and also better resistance than ordinary ASTM 316/316L steels with 2.1% Mo.

Conclusions

Laboratory and field tests established that bright-annealed and surface-finished heat-treatment processes hold distinct advantages in manufacturing tube for severe and chloridic hydraulic applications. Furthermore, enhancements to levels of chromium, nickel, and molybdenum improve these steels’ mechanical strength, corrosion resistance, and life over those of equivalent standard grades, such like ASTM 316L.

For these reasons, Sandvik SAF 2507 is widely used in highly corrosive conditions for hydraulic and instrumentation applications in tropical marine environments. Sandvik 3R60 is relied upon for a wide range of industrial applications where steels of type ASTM 304 and 304L have insufficient corrosion resistance such as hydraulic systems, heat exchangers, condensers, and pipelines.

Eduardo Perea is Global Technical Marketing manager for Sandvik Materials Technology, Sandviken, Sweden. For more information, visit www.smt.sandvik.com.

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