An increasing proportion of the products of petroleum and natural gas contain wet carbonic acid gas or hydrogen sulfide gas. It is well known that carbon steels and low alloy steels are subjected to heavy corrosion in such wet environments. Therefore, in the transport of corroding petroleum and natural gas, corrosion inhibitors have been conventionally used to protect steel pipes from corrosion. However, it is becoming more and more difficult to use corrosion inhibitors in offshore oil wells because of the vast expenditure required for the addition and recovery thereof and because of sea pollution. There is thus an increasing need for corrosion resistant materials requiring no additional corrosion inhibitors.
To this end, there have been many proposals for steels or steel pipes having good corrosion resistance to carbonic acid-containing environments or the like and also having good weldability. Generally, the proposed steels and steel pipes contain about 11 to 15 wt % Cr to achieve the corrosion resistance to carbonic acid-containing environments, have a reduced C content to provide an improved weldability, and are heat-treated by quenching and tempering to establish a microstructure of tempered martensite to ensure good strength and toughness. For example, Japanese Unexamined Patent Publication (Kokai) No. 4-99154 and Japanese Unexamined Patent Publication (Kokai) No. 4-99155 disclose high Cr steels for pipelines in which the C and N contents are reduced and substitutional elements, such as an austenite stabilizer, are added to ensure good weldability.
Pipelines and pressure vessels are joined or produced by welding. However, there were no filler materials and welding methods applicable to the above recited high Cr weldable steels. "NKK Technical Report", 1989, No. 129, pages 15-22 reports that AISI 410 steel was produced in the form of a UOE pipe and a TIG-welded joint (corresponding to that produced in field welding of a pipeline along the circumferential profile line thereof) was formed using a filler material corresponding in composition to the base material with additional Ni. However, as can be seen from the publication recited above, a filler material corresponding in composition to a high Cr steel, even though containing a large amount of Ni, produces a weld metal having a very high hardness. The weld metal is also very susceptible to low temperature cracking and absolutely requires pre- or post-weld heat treatment. Therefore, it is practically difficult to weld high Cr steels by using a composition-corresponding filler material or a filler material of a martensitic stainless steel.
On the other hand, when high Ni austenitic stainless steels having good corrosion resistance and Ni-based super alloys are used as a filler material, the weld metal does not cause preferential corrosion thereof, has a low hardness, and ensures good toughness of a weld metal. However, austenitic stainless steels and Ni-based super alloys have a drawback in a low strength because of the crystal structures thereof. If a weld metal has a very low strength, an external force may cause a concentrated deformation at the weld metal leading to a detrimental fracture (as characterized by the term "under-matching"). Therefore, it was also very difficult to perform welding of high Cr steels using a filler material of austenitic stainless steel or high Ni alloy. In recent years, dual-phase stainless steels were also used as a filler material but they mostly caused under-matching because the weld metal had a low strength.