It is a truth universally acknowledged that API 5l steel pipe are marked by low alloy structures with a carbon contend of less than two point five percent and it has been observed that API steel pipe metal micro structure is a complex mixture of more than two constituents, such as ferrite, polygonal ferrite, aligned plate as well as aggregate ferrite, which means that when alloying elements are added to the API 5l steel pipe metal, a variety of micro constituent layers may be formed at the same time.
What is more, when thick plates are used to produce API steel pipe, a high level of residual stresses will be generated, and it is usual to perform a stress relieve heat treatment in the process, which is always done at temperatures between seven and six hundred degrees. Therefore it does not change significantly the micro structure and mechanical properties of API 5l steel pipe base metals, and in some few cases, when the steel is hot or cold worked, it is necessary to perform a normalizing heat treatment of the API steel pipe in order to recover its original mechanical properties as required.
It has been proven that as normalizing involves heating above eight hundred degrees in order to promote the API steel pipe base metal grain refinement, this will change the original characteristics of the API steel pipe as piping structure since the effect of the API steel pipe normalizing heat treatment on the metal micro structure has not been yet well studied in the world in spite of the fact that there are many evaluations of API steel pipe in the field. According to scientific research results, the API steel pipe normalizing heat treatment will occur at a rate of two hundred degrees per hour from ambient temperature up to more than nine hundred degrees and will remain at this temperature for over two hours. Afterwards, on top of that, API steel pipe will be removed from the oven and air cooled up to the ambient temperature after which quantitative evaluation of the pipe will be carried out only on the metal conditions.
