Steels ductile/brittle transition

With the precipitation hardening types, high strengths can be obtained with good toughness. A feature of these steels is that the ductile-brittle transition is less sharp, although low impact values are obtained at very low temperatures. Properties for a typical example are shown in Tables 3.17 and Fig. 3.13. 

Steels are normally ductile at ambient temperatures, although they are often close to brittle behaviour, as is indicated by the ductile-brittle transition temperature. If the conditions at the tip of a sharp crack are considered, it can be seen that brittle fracture will occur if it is easier to break the atomic bond at the tip of the crack than it is to emit a dislocation to blunt the crack (see Thompson and Lin ). As dislocation emission is more temperature sensitive than the bond strength it becomes more difficult at low temperatures and brittle fracture occurs. The very severe effects of hydrogen on the performance of steels can be attributed to its role in allowing brittle fracture... 

To avoid brittle fracture during operation, maintenance, transportation, erection, and testing, good design practice shall be followed in the selection of fabrication methods, welding procedures, and materials for vendor furnished steel pressure retaining parts that may be subjected to temperature below the ductile-brittle transition point. 

NOTE Good design practice should be followed in the selection of fabrication methods, welding procedures, and materials for vendor-furnished steel pressureretalning parts that may be subject to temperatures below the ductile-brittle transition temperature. The published design-allowable stresses for metallic materials in internationally recognised standards such as the ASME Code and ANSI standards are based on minimum tensile properties. Some standards do not differentiate between rimmed, semi-killed, fully killed hot-rolled and normalised material, nor do they take into account whether materials were produced under fine- or course-grain practices. The vendor should exercise caution in the selection of materials intended for services between 0 °C (-20 °F) and 40 °C (100 °F). 

Materials such as austenitic stainless steels, nickel-based alloys, and titanium alloys can be used as materials for pressure vessel components in cryogenic applications at temperatures as low as 200°C. Alloy steels have brittle transition points making their impact properties at low temperatures unsuitable for pressure applications. Closures and bolts must also be made of materials that remain ductile at low temperatures. 

Fig. 7. Effect of processing techniques on the suppression of ductile-brittle transition temperature of an Fe-12Mn-0.2Ti alloy steel...

Low temperature Low temperatures lead to a penalty if carbon steels or other metals are exposed to temperatures at or below their ductile/brittle transition temperatures. A factor of 0.2 or 0.3 is assigned. 

D R Harries, R W Nichols and C Judge,The effect of neutron irradiation on the ductile-brittle transition temperature of steels and its relevance to reactor pressure vessels. Symposium on Steels for Reactor Pressure Circuits, Iron and Steel Institute Special Report, No 69,1961,297-327. 

Nil Ductility Transition Temperature (NDT) Also known as DBTT (ductile-brittle transition temperature) The temperature above which the material is predominantly ductile and below which it is predominantly brittle. The NDT represents the point at which the fracture energy passes below a predetermined point, i.e. 15 ft-lbs (20 joules) for ordinary steel or 40 ft-lbs (54 joules) for Cr-Mo steels. 

The Charpy-V impact data are presented in Table 4.5. Energy to fracture (EF) and brittle fracture area (BFA) at 20 °C are mentioned. It can be seen that the steel was embrittled during service. Some after-service specimens showed 50% of brittle fracture area, indicating 20 °C to be the ductile-brittle transition temperature of the material. 

It is known that phosphorus exists in steel in the form of solid solution, which can significantly improve strength and hardness of steel at room temperature. However, die plasticity and toughness of steel, especially the low temperature plasticity is sharply reduced and leads to high ductile brittle transition temperature. This phenomenon is commonly known as cold brittleness of steel [1-3]. Hence, phosphorus should be removed as much as possible in the steehnaking process, because it is generally harmfiil to the iron and steel products. 

Effect of Copper. Residual elements, Cu especially, seriously affect the sensitivity of steel to irradiation. Experiments on iron, simple and complex iron alloys, and pressure vessel steels have shown that irradiation embrittlement is markedly increased in the presence of Cu [58,59]. It was concluded that a Cu content of more than about 0.1 wt% has a determinable effect, although there are some indications that such an effect may be found with Cu contents as low as 0.03 wt% [60]. Increasing the Cu content of irradiated RPV steel can cause an irradiation-induced shift in the ductile-brittle transition temperature ATT (see Figure 4.20). 

Two main advantages of tantalum are that its anodic film has better dielectric properties than aluminum, and it has a very low ductile-brittle transition temperature. Tantalum also has a versatile aqueous corrosion resistance. In most environments, tantalum is comparable to glass in corrosion resistance, whereas it has physical and mechanical properties similar to mild steel. Tantalum is also resistant to attack by many liquid metals such as Li<1000°C, Na, K NaK <1000°C, ThMg <850 = C, U <1400°C, Zn <450°C, Pb <850°C, Bi <500°C, and Hg <600°C. 

To demonstrate the effectiveness of the various tests in producing a ductile-brittle transition, tests were also made on plate and welds of a typical ductile-brittle material—plain carbon steel. The steel used was SAE1034 steel of the composition shown in Table 111. 

In the four types of tests discussed above, a ductile—brittle transition was found under every set of test conditions in SAE1034 steel, a typical ductile-brittle metal. With the most severe tests, those involving impact loading and notches, the transition temperature zone of the SAE1034 steel plate was found to lie above 0 F. No ductile—brittle transition was found in plate of aluminum alloy 5083-H113, or in weld deposits of aluminum alloy 5183, under any of the test conditions, at temperatures to -320 F. 

Fig. 14. Effect of simultaneous addition of rare earth with boron on the ductile-brittle transition temperature of Fe-0.12C-0.25Si-1.50Mn-0.006N steel.

Fig. 24. Effect of rare earth content on the ductile-brittle transition temperature of 19Cr-2Mo steel welded in Ar+0.5%air.

The body-centered-cuhic (bcc) metals and alloys are normally classified as undesirable for low temperature construction. This class includes Fe, the martensitic steels (low carbon and the 400-series stainless steels). Mo, and Nb. If not brittle at room temperature, these materials exhibit a ductile-to-brittle transition at low temperatures. Cold working of some steels, in particular, can induce the austenite-to-martensite transition. 

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