{"id":9576,"date":"2018-05-31T14:08:38","date_gmt":"2018-05-31T19:08:38","guid":{"rendered":"http:\/\/blog.wika.com\/us\/\/?p=9576"},"modified":"2023-08-25T02:26:31","modified_gmt":"2023-08-25T07:26:31","slug":"grain-growth-type-k-thermocouples","status":"publish","type":"post","link":"https:\/\/blog.wika.com\/us\/products\/temperature-products\/grain-growth-type-k-thermocouples\/","title":{"rendered":"Grain Growth in Type K Thermocouples: Some Causes and Solutions"},"content":{"rendered":"

Extremely high temperatures over time lead to grain growth in type K thermocouples, which can cause metal separation and thermocouple failure. Fortunately, good solutions exist for minimizing the effect of grain growth.<\/strong><\/p>\n

Grain growth is a common cause of failure in type K thermocouples<\/a>. In applications such as tubeskin temperature measurement in fired process furnaces, the temperature measurement device is exposed to process gas and radiant energy, creating temperatures that are much higher than the tubeskin temperature measuring junction. Elevated temperature over time is the overriding factor in grain growth that ultimately damages the thermocouple.<\/p>\n

The Causes and Effects of Grain Growth<\/h2>\n\n

Example of a metal’s crystal structure<\/p><\/div>\n \n

Grain growth is a condition where a material\u2019s crystallites, or grains, become larger. In thermocouples, it is a failure mode that has to do with a metal\u2019s grain structure, different coefficients of expansion, and an application\u2019s high temperatures \u2013 above 1,200\u00b0<\/span>F (649\u00b0C) for type K.<\/p>\n

A sheathed thermocouple contains three different types of metals: one in the positive conductor, another in the negative conductor, and a third type in the sheath. The sheath is usually made of stainless steel or another alloy, while each <\/span>type of thermocouple<\/span><\/a>has a specific pairing of metal conductors. Type K, the most commonly used thermocouple, has a NiCr (Chromel\u00ae) positive leg and a NiAl (Alumel\u00ae) negative leg.<\/span><\/p>\n

Each metal has a different coefficient of expansion \u2013 stainless steel has the largest, and the negative leg has the smallest. As a result, every time a type K thermocouple is exposed to high temperatures, the negative conductor leg undergoes stress. The crystal structure of NiAl goes from having many small grains (worked or manufactured state) to a few large grains (relaxed state). The large crystal structure has less mechanical \u201croughness\u201d and fewer \u201cledges,\u201d causing the negative conductor to actually separate when stressed by the differential expansion. <\/span>Small conductors are also more susceptible to grain growth than large ones.<\/p>\n

Grain growth and failure in type K thermocouples has been well established. In one test, type K thermocouples sheathed in 310 stainless steel with a \u00bc\u201doutside diameter (OD) were subjected constantly to temperatures between 2,000\u00b0<\/span>F and 2,100\u00b0<\/span>F (1,093\u00b0C and 1,149\u00b0C). All samples showed extensive grain growth of two to five grains in cross-sections of the Alumel wire over time, though not all of them failed. The failure rates over time:<\/p>\n