Markus O. Speidel

Metallurgy: High nickel release from 1- and 2-euro coins

The amount of nickel is regulated in European products that come into direct and prolonged contact with human skin because this metal may cause contact allergy, particularly hand eczema. Here we show that 1- and 2-euro coins induce positive skin-test reactions in sensitized individuals and release 240–320-fold more nickel than is allowed under the European Union Nickel Directive. A factor contributing to this high release of nickel is corrosion due to the bimetallic structure of these coins, which generates a galvanic potential of 30–40 mV in human sweat.

Stress corrosion cracking of stainless steels in NaCl solutions

The metallurgical influences on the stress corrosion resistance of many commercial stainless steels have been studied using the fracture mechanics approach. The straight-chromium ferritic stainless steels, two-phase ferritic-austenitic stainless steels and high-nickel solid solutions (like alloys 800 and 600) investigated are all fully resistant to stress corrosion cracking at stress intensity (K1) levels ≤ MN • m-3/2 in 22 pct NaCl solutions at 105 °C. Martensitic stainless steels, austenitic stainless steels and precipitation hardened superalloys, all with about 18 pct chromium, may be highly susceptible to stress corrosion cracking, depending on heat treatment and other alloying elements. Molybdenum additions improve the stress corrosion cracking resistance of austenitic stainless steels significantly. The fracture mechanics approach to stress corrosion testing of stainless steels yields results which are consistent with both the service experience and the results from testing with smooth specimens. In particular, the well known “Copson curve” is reproduced by plotting the stress corrosion threshold stress intensity (ATISCC) vs the nickel content of stainless steels with about 18 pct chromium.

Clean steels for steam turbine rotors—their stress corrosion cracking resistance

This work presents the results of a comprehensive study concerning stress corrosion crack growth rates in steam turbine rotor steels exposed to hot water. The effects of stress intensity, temperature, and dissolved gases in the water have been investigated. Special attention has been given to the influence of impurities and alloying elements in the steel such as P, S, Mn, Si, Mo, and Ni, and to the effect of yield strength and fracture toughness on the growth rates of stress corrosion cracks. The results of this study clearly show that there exists a threshold stress intensity of about 20 MNm−3/2 above which the invariably intergranular stress corrosion cracks grow at a constant, stress-independent velocity. This plateau stress corrosion crack growth rate isnot affected by the oxygen and carbon dioxide concentration in the water. The temperature and the yield strength of the steel have a strong influence on the growth rate of stress corrosion cracks. In contrast, there isno effect of the steel composition within the range investigated, neither of the impurity elements such as P and S, nor of the major alloying elements such as Mn, Si, Mo, and Ni. Steels with low fracture toughness due to temper embrittlement do not exhibit faster stress corrosion crack growth rates in water than nonembrittled steels. No direct relationship between intergranular temper embrittlement and intergranular stress corrosion crack growth in water can be demonstrated.

Nickel and Chromium-Based High Nitrogen Alloys

High nitrogen stainless steels (HNS) were developed in the second half, and particularly in the last quarter, of the twentieth century. Most of these steels are either free of nickel or contain very small amounts of nickel. The present article takes a different approach. Here we consider austenitic solid solutions with facecentered cubic crystal lattice that are rich in both nickel and nitrogen. This is not an immediately obvious group of materials since nickel and nitrogen are often considered to be mutually exclusive alloying elements; it is well known that nickel reduces the solubility of nitrogen in steel melts. However, in the present article we show that high nitrogen austenitic solid solutions are both possible and useful with nickel contents of 10 to 20 weight–percent and nickel contents with more than 30 weight–percent, right up to nickel-base austenitic solid solutions, as long as enough chromium is added as an alloying element.

Thermomechanical Strengthening of Nitrogen-Bearing Austenitic and Duplex Stainless Steels

Austenitic stainless steels are widely used in industry because of their superior corrosion resistance and high (especially ductility and toughness) mechanical properties. Relatively low yield strength, however, is an obstacle to the spread of this application. The typical strengthening methods of austenitic stainless steels include solid solution hardening and grain refinement. Two methods, i.e., solid solution hardening by nitrogen addition and work hardening by thermomechanical processing (TMP) seem to be most effective in increasing the strength of hotdeformed steels. Thermomechanical processing can give austenitic stainless steel grain refinement and substructure hardening that raise strength without much

Austenitic stainless steels of high strength and ductility

Abstract High strength and high ductility together are prime requirements for modern structural metallic materials. Important industrial examples are steels for car components such as body sheets, tubes and forgings. The present paper traces two lines of alloy development, stable austenitic stainless steels and intentionally metastable austenitic stainless steels. Although both groups have different advantages and disadvantages, each one exceeds all other presently available structural metallic materials in terms of strength to density ratio at equal ductility.