Seam Annealing of HF Welded API Pipe
Dr. Paul Scott
Vice President, Technology
Thermatool Corp
East Haven, CT, USA
健身教练学习网Why Seam Anneal?
Seam annealing is an important in-line process for making high frequency (HF) welded pipe to API 5L or 5CT specifications. Its purpo is to produce pipe compliant with the following API requirements:
jago• From API 5L, Section 5.1.3.3.1 Electric Welded Pipe: For grades higher than X42, the weld am and the entire heat affected zone shall be heat treated so as to simulate a normalizing heat treatment…
Also:
For grades X42 and lower, the weld am shall be similarly heat treated,
or the pipe shall be procesd in such a manner that no untempered
martensite remains.ebx
• From API 5 CT, Section 6.2.1 Heat Treating – General: The weld am of electric-welded pipe shall be heat-treated after welding to a minimum
temperature of 540°C (1000°F) or procesd in such a manner that no
我的母亲胡适pptuntempered martensite remains.
Induction am annealing is the most widely ud method for either normalizing the am of HF welded pipe or removing its untempered martensitic structure. The HF Pipe Making Process and a Little Metallurgy:
HF welded pipe is fabricated by forming strip steel into a tubular shape using a roll forming mill and then welding the edges together. This process is illustrated in Figure 1. The welding is done by using either electrical contacts or an induction coil to create a high frequency electric current (genera
lly between 100 kHz and 400 kHz) along the strip edges. Becau of the nature of high frequency electrical currents, a physical phenomenon called the “Proximity Effect” forces most of the current to flow on the strip’s edges. This caus the edges to heat to the forge welding temperature and a t of “squeeze” rolls force the edges together under sufficient pressure to produce a forge weld. The weld is then allowed to cool at a relatively high rate to insure that it has good integrity. The result is, that while the weld is a forge weld with no “cast” structure, the metallurgical properties in the Heat Affected Zone (HAZ) are now different than tho of the rest of the pipe.
Figure 1 The HF Pipe Making Process a. Typical roll forming type tube mill ud to produce API pipe b. “Flower Pattern” Diagram showing how the forming rolls successively shape the strip into a pipe. c. HF Weld Area showing the induction work coil and the weld forging roll boxbid
Let’s look in greater detail at the transformation that occurs in the weld zone. Seam welded API product is made from various high tensile steels that have a carbon content between about 0.05% a
nd 0.26% for 5L products and between about 0.1% and 0.5% for 5CT products. When the steels are heated to the forge welding temperature, they achieve a maximum of about 1500 Degrees C (2750 F) although most of the metal at this temperature is squeezed out from the weld zone during the forging process. From this maximum, the temperature decreas as one moves circumferentially away from the weld bond plane. Beginning at the temperature of 721 degrees C (1330 F) to a temperature of about 850 degrees C (1560 F), the iron’s crystalline lattice in the steel undergoes a radical transformation from having a body centered or Ferrite crystalline structure to having a face centered or Austenite crystalline structure. This is illustrated in Figure 2. When this happens, the carbon atoms from the dissolved carbides, which are much smaller than the iron atoms, move into the Austenitic structure to fill the “gaps” between the iron atoms. If the steel is cooled fast enough, as it is after it is forge welded, the iron crystalline lattice undergoes the rever transformation from the Austenite structure to the Ferrite structure faster than the carbon atoms can escape from their positions between the iron atoms in the crystalline lattice. Hence, they become trapped in the iron lattice. This results in a body centered tetragonal crystalline structure called Martensite. With Martensite, the carbon atoms trapped between the iron atoms put the crystal lattice under great strain. Thus the steel in this region is hard and brittle. This is an undesirable condition and hence the API requirements to eliminate the untempered Martensite. Samples of a weld am before and after normalizing are shown in Figure 3.
caramel>质量管理信息系统
Weld am before
normalizing – Change in
metallurgical structure is
quite visible
Weld am after断章取义英文
normalizing – No visible
change in metallurgical
特朗普和希拉里
padstructure.
Figure 3 – Weld Seam Before and After Seam Normalizing
The Metallurgical Effects of “Seam Annealing”
While the process is called “am annealing”, this is a misnomer. It is really a normalizing or tempering type metallurgical process. To achieve the API specifications, the carbon atoms must be freed from the iron’s crystalline lattice. To do this, the weld zone must be reheated and then cooled slowly enough so that the carbon atoms can escape.
Tempering is a time versus temperature process. The hotter the temperature of the steel, the faster the carbon atoms will escape from the lattice. Tempering occurs at temperatures lower than the transition temperature between Ferrite and Austenite and in practice is performed between 540 degrees C (1000 F) and 700 degrees C (1300 F).
To normalize the weld am, the steel is treated to a temperature that retransforms it to Austenite (See Figure 4). It is then allowed to cool very slowly so that it fully returns to a Ferrite and carbine co
mposition. In practice, this means heating the weld am to between 950 degrees C (1750 F) and 1010 degrees C (1850 F). It is important to note, however, that exceeding 1010 degrees C (1850 F), and certainly 1100 degrees C (2000 F), caus coarning of the grains of metal in the weld area (grain growth) creating a zone of weakness and potential for accelerated corrosion.
Figure 4 – Steel Pha Diagram and a Function of Carbon Content Finally, the trend in line pipe materials is toward higher yield steels (80 to 100 kpsi) and many of the are very low carbon, micro-alloyed, thermo mechanically procesd (TMP) steels. In this process, the steel slabs are finish rolled at temperatures just above the Austenite formation temperature and then rapidly cooled on a laminar flow table. Rolling at this relatively low temperature crushes the Austenite the result is a very fine-grained Ferrite. The weld am for the materials cannot be treated with the conventional am annealing process becau the fine-grain structure is totally destroyed in the regions where the temperature has been raid high enough to convert the crystalline lattice to Austenite. New quench and temper annealing process are being developed to deal with the materials and will be an important area for further rearch.
How Seam Annealing is Accomplished:
Seam annealing is performed as an inline part of the pipe making process. A t of medium frequency induction heaters and a reasonably long air-cooling zone are inrted in the HF pipe production line between the weld area and the final pipe sizing roll stands. The induction am heaters rai the temperature of the weld zone to between about 950 degrees C (1750 F) and 1010 degrees C (1850 F) as measured by an optical pyrometer aimed at the outside center of the weld
am. The pipe is then allowed to cool slowly as it pass through the air-cooling zone. After it cools to below 700 degrees C (1300 F) no metallurgical changes will take place if it is water quenched. However, in practice, it is generally air cooled to below 370 degrees C (700 F) to avoid quench cracking. Below this temperature, the steel can be safely water quenched before passing though the mill’s final pipe sizing ction. An example of a typical am annealing station is shown in Figure 5.
