Temper embrittlement is a type of brittle failure that can occur in steels, particularly in low alloy steels, when they are exposed to certain temperature ranges for prolonged periods. This phenomenon can significantly compromise the material’s mechanical properties, especially its toughness and ductility. When steel undergoes temper embrittlement, it becomes prone to brittle fracture, potentially leading to catastrophic failure.
Fortunately, modern steels are far less vulnerable to temper embrittlement, thanks to extensive research on the phenomenon and improved control over the presence of problematic “tramp” elements. These advancements have significantly reduced the risk compared to older steel grades. However, in cases where materials produced before the early 1970s are still in use and in weldments, the risk of temper embrittlement remains a concern.
Temper embrittlement occurs when certain low-alloy steels, primarily the 2.25Cr-1Mo low-alloy steel, 3Cr-1Mo (to a lesser extent), and the HSLA Cr-Mo-V rotor steels are exposed to temperatures between approximately 345°C to 575°C for prolonged periods. During this time, specific elements in the steel, such as phosphorus, antimony, arsenic, and tin, migrate to grain boundaries and form brittle phases, which makes the steel more susceptible to brittle fracture. This change causes an upward shift in the ductile-to-brittle transition temperature as measured by Charpy impact testing. Although the loss of toughness is not evident at operating temperature, equipment that is temper embrittled may be susceptible to brittle fracture during start-up and shutdown.
Several factors play a crucial role in the development of temper embrittlement. First and foremost, the alloy steel composition, thermal history, metal temperature, and exposure time are all critical elements that contribute to the likelihood of embrittlement. The steel’s specific makeup and the conditions it experiences during use dictate how susceptible it will be to this form of damage.
Susceptibility to temper embrittlement is largely determined by the presence of the alloying elements manganese and silicon and the tramp elements phosphorus, tin, antimony, and arsenic. Additionally, factors like the strength level of the steel and its heat treatment/fabrication history can also impact its vulnerability to embrittlement.
Temper embrittlement is particularly problematic for steels like 2.25Cr-1Mo, which are commonly used in high-pressure and high-temperature applications. Studies show that temper embrittlement in these steels occurs more rapidly at temperatures around 480°C compared to the 425°C to 440°C range. However, the damage caused at the higher temperatures is more severe, especially after prolonged exposure.
SEM Image detailing intergranular fracture of a pipe section which failed due to temper embrittlement.
While some embrittlement may occur during fabrication heat treatments, the most significant damage typically happens over many years of service when the material is exposed to the embrittling temperature range. The effects of temper embrittlement can severely compromise the structural integrity of a component, particularly if it contains a crack-like flaw. In such cases, evaluating the material toughness is essential, especially when assessing the severity of operating conditions like pressure and stress. This is especially important in hydrogen service, where the material is subjected to additional stresses that may exacerbate the embrittlement process.
One of the key indicators of temper embrittlement is a noticeable upward shift in the ductile-to-brittle transition temperature. The ductile to brittle transition temperature can be raised by as much as 100°C. This can be observed in a Charpy V-Notch (CVN) impact test, where the transition temperature of embrittled material is higher than that of the non-embrittled or de-embrittled material. The increase in this transition temperature signifies that the material has become more brittle and is more likely to fail under stress at lower temperatures. Importantly, temper embrittlement does not affect the upper shelf energy, meaning the material’s ability to absorb high-energy impacts remains unchanged.
Digital microscope Image detailing intergranular cracking of a pipe section which failed due to temper embrittlement.
Prevention
a) Existing Materials
Temper embrittlement is a condition that cannot be completely prevented if the material contains critical levels of embrittling impurity elements and is exposed to the embrittling temperature range. Once materials are susceptible, specific strategies can be implemented to minimise the risk of brittle fracture.
The approach to reduce the potential for brittle fracture during start-up and shutdown is the use of a pressurisation sequence. By limiting the system pressure to about 25% of the maximum design pressure when temperatures are below the minimum pressurisation temperature (MPT), the risk of embrittlement-related failure can be minimised.
The MPT is not always a single temperature but can be represented by a pressure-temperature curve that outlines safe operating conditions to reduce the chances of brittle fracture. MPTs typically range from 170°C for highly temper-embrittled steels to 50°C or lower for newer steels that are resistant to temper embrittlement.
In addition to controlling pressure and temperature during start-up and shutdown, hydrotesting of susceptible equipment should be approached cautiously. The risk of brittle fracture can be minimised by considering the effects of temper embrittlement when conducting such tests.
For weld repairs, the effects of temper embrittlement can be temporarily reversed (de-embrittled) by heating the material at 1150°F (620°C) for 2 hours per 1 inch (25 mm) of thickness and then cooling it rapidly to room temperature. However, it’s essential to note that re-embrittlement can occur over time if the material is re-exposed to the embrittling temperature range.
b) New Materials
The most effective way to minimise the likelihood and severity of temper embrittlement is to control the composition of the steel, especially the levels of certain elements that contribute to embrittlement. Limiting the presence of manganese, silicon, phosphorus, tin, antimony, and arsenic in both the base metal and welding consumables is critical. Moreover, specifying and controlling the strength levels and Post-Weld Heat Treatment (PWHT) procedures can further reduce the risk of temper embrittlement.
A practical approach to minimising temper embrittlement involves controlling two important factors: the J factor for the base metal and the X factor for the weld metal. These factors are based on the material composition and are defined as follows:
- J factor:
J= (Si + Mn) x (P + Sn) x 10^4 (elements in wt%) - X factor:
X= (10P + 5Sb + 4Sn + As)/100 (elements in ppm)
For 2.25 Cr steel, the typical maximum values for the J and X factors are 100 and 15, respectively. Studies have shown that limiting the (P + Sn) content to less than 0.01% is effective in minimising temper embrittlement because (Si + Mn) primarily control the rate of embrittlement.
Another newer, albeit less widely used, factor is the Equivalent Phosphorus (P) content, which takes into account the combined effects of various alloying elements. This is defined as:
P = C + Mn + (Mo+Cr)/3 + Si/4 + 3.5 x [(10 x P) + (5 x Sb) + (4 x Sn) + As] (elements in wt %).
Detection
While inspection is not typically used to detect temper embrittlement directly, maintaining awareness of susceptible equipment is crucial for preventing future damage. Temper embrittlement is a metallurgical change that does not present obvious signs, such as visible cracking, until the material has already become brittle and prone to fracture. Therefore, the key to mitigating the risks of temper embrittlement lies in being proactive about identifying and monitoring materials that may be susceptible based on their composition and exposure conditions.
One effective method of monitoring temper embrittlement is to install blocks of original heats of alloy steel material inside the reactor or other high-risk areas. These test blocks are made from the same alloy as the equipment being used, providing a representative sample of how the material may behave under operational conditions. Periodically, samples are removed from these blocks for impact testing, specifically to establish and track the ductile-to-brittle transition temperature. This temperature shift can be an early indicator of temper embrittlement, as the material’s ability to withstand impact will decrease as it becomes more embrittled.
For more information or expert advice on temper embrittlement and its prevention, feel free to contact us at info@r-techmaterials.com. Our team of specialists can assist with material integrity challenges and provide support to ensure the safety and longevity of your engineering systems.