Abstract:
For stress-crack-sensitive materials, a weld pool is formed on a substrate. The weld pool is simultaneously plastically strained and cooled while the temperature of the hot metal weld pool lies substantially within a predetermined range in which hot cracking typically occurs. The hot metal is plastically strained and cooled until the temperature of the weld material is below the predetermined temperature range associated with hot cracking or until residual stresses in the weld are sufficiently low to preclude cracking in the completed weld.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates generally to welding and particularly to welding on stress-crack sensitive materials to reduce or eliminate hot cracking and reduce or eliminate stress corrosion cracking.  
           [0002]    There are various types of conventional fusion welding processes, such as electric arc, laser beam, or electron beam welding. In those processes, a molten pool of hot metal is formed, either by melting a substrate or adding a filler metal, or both. Materials, however, are oftentimes sensitive to hot cracking. Hot cracking of the welded surface is typically caused by strains and stresses due to contraction on cooling, i.e., during the phase change from liquid hot metal to a solid state. An extreme but actual example of hot cracking sensitive materials is fusion welding on material containing higher levels of helium, such as in permanent portions of older nuclear reactor vessel internals near the fuel core. In neutron irradiated austenitic stainless steels with significant boron content (which is susceptible to transmutation to helium), the helium in the weld materials causes several adverse effects through changes in mechanical properties. For example, when high helium content materials are exposed to the heat of a welding cycle, the high temperature allows the helium to diffuse rapidly to grain boundaries which form voids which, in turn, weaken the material resulting in hot cracking. Even for known low heat input fusion welding processes, the capability to reliably weld without hot cracking is limited to materials having relatively low helium levels. Hot cracking is also not limited to materials having a helium content but constitutes only one type of material in which hot cracking occurs. The hot cracking problem is also compounded by the typically high tensile temporal and residual surface stresses caused by the fusion processes. This adverse stress situation in the as-welded condition is characteristic of all conventional fusion welding processes and applications, especially for the heavy section thicknesses of materials generally found in permanent nuclear vessel internals and for the vessel wall itself or its attachments. It is effectively impossible to provide sufficiently low heat in the fusion welding process to avoid hot cracking, while still having a viable fusion welding process.  
           [0003]    In addition to the hot cracking problem during cooling of the weld pool, stress corrosion cracking (SCC) can occur in materials susceptible to thermal or neutron sensitization when used in aggressive environments such as oxygen or halogen containing high temperature nuclear reactor water or moderator. This type of environmentally induced cracking occurs when the level of surface residual stress becomes sufficiently tensile as is typically the case for conventional fusion welding practice.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0004]    In accordance with a preferred embodiment of the present invention, there is provided apparatus and methods for welding on stress crack sensitive materials which minimizes or eliminates the problem of stress cracking in the resulting weld. To accomplish the foregoing, hot viscous or plastic weld material is formed on a solid substrate by conventional fusion welding apparatus, typically including a heat source, and which may include the addition and melting of filler material as well as melting of a portion of the substrate. The process includes simultaneous hot compression with or without concurrent heat sinking of the weld. Preferably, compression force and heat sinking are simultaneously applied to the hot weld deposit during progression of the weld pass.  
           [0005]    More particularly, plastic deformation of the hot weld metal is provided by controlled contact with a compression tool. External cooling, in addition to the inherent internal conduction to the hot surrounding material, is simultaneously provided to the weld area by contact with the conductive end of the compression tool, while the pressure of the compression tool is maintained against the weld pool. The predetermined compression and external heat sinking conditions are maintained on the weld area until the weld material is known or measured to be below its predetermined hot crack sensitive temperature or until the residual stress is sufficiently low to preclude in-service cracking. Additional cooling may be applied once outside the range of temperatures at which hot cracking is anticipated to occur, with the result that the weld surface has reduced tensile and preferably compressive stresses.  
           [0006]    The foregoing method is also applicable to minimizing or eliminating post-weld stress corrosion cracking. By plastically compressively stressing the hot weld material when the hot weld material lies within a predetermined temperature range corresponding to the hot crack-sensitive predetermined temperature range, any tendency toward stress corrosion cracking in the completed weld is minimized or eliminated.  
           [0007]    In a preferred embodiment according to the present invention, there is provided a method of welding materials while the materials are in a hot crack-sensitive predetermined temperature range comprising the steps of (a) forming hot metal weld material on a portion of a substrate, (b) plastically compressively stressing the hot metal weld material from an external source while the temperature of the hot metal weld material lies substantially within the predetermined temperature range and (c) performing step (b) until the temperature of the weld material is below the predetermined temperature range or until residual stresses are sufficiently low to minimize or preclude hot cracking in the completed weld.  
           [0008]    In a further preferred embodiment according to the present invention, there is provided a method of welding materials while the materials are in a crack-sensitive predetermined temperature range comprising the steps of (a) forming hot metal weld material on a portion of a substrate, (b) simultaneously plastically straining and cooling the hot metal weld material from an external source while the temperature of the hot metal weld material lies substantially within the predetermined temperature range and (c) performing step (b) until the temperature of the weld material is below the predetermined temperature range or until residual stresses are sufficiently low to preclude cracking in the completed weld.  
           [0009]    In a further preferred embodiment according to the present invention, there is provided a method of welding materials while the materials are in a predetermined temperature range to minimize or eliminate post-weld stress corrosion cracking comprising the steps of (a) forming hot metal weld material on a portion of a substrate, (b) plastically compressively stressing the hot metal weld material from an external source while the temperature of the hot metal weld material lies substantially within the predetermined temperature range and (c) performing step (b) until the temperature of the weld material is below the predetermined temperature range or until residual stresses are sufficiently low to minimize or preclude stress corrosion cracking in the completed weld.  
           [0010]    In a further preferred embodiment according to the present invention, there is provided a method of welding materials while the materials are in a predetermined temperature range to minimize or eliminate post-weld stress corrosion cracking comprising the steps of (a) forming hot metal weld material on a portion of a substrate, (b) simultaneously plastically compressively stressing and cooling the hot metal weld material from an external source while the temperature of the hot metal weld material lies substantially within the predetermined temperature range and (c) performing step (b) until the temperature of the weld material is below the predetermined temperature range or until residual stresses are sufficiently low to minimize or preclude cracking in the completed weld. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a schematic illustration of a combined compression and heat sinking tool for welding stress crack sensitive materials according to a preferred embodiment of the present invention;  
         [0012]    [0012]FIG. 2 is a view similar to FIG. 1 illustrating an integrated welding and deforming assembly;  
         [0013]    [0013]FIG. 3 is a perspective view of an embodiment of the invention employed for underwater welding;  
         [0014]    [0014]FIG. 4 is a schematic illustration similar to FIG. 1 showing a further embodiment of the present invention;  
         [0015]    [0015]FIG. 5 is a schematic illustration similar to FIG. 1 of a compression and heat sinking tool for continuous application of point-wise surface and heat sinking;  
         [0016]    [0016]FIG. 6 is an enlarged fragmentary perspective view of the portion of the roller in FIG. 5;  
         [0017]    [0017]FIG. 7 is a schematic representation similar to FIG. 1 of a compression and heat sinking tool for continuous application of an area-wide surface compression and heat sinking;  
         [0018]    [0018]FIG. 8 is an enlarged fragmentary cross-sectional view of a portion of the roller of FIG. 7;  
         [0019]    [0019]FIG. 9 is a schematic illustration similar to FIG. 1 illustrating a compression and heat sinking tool for intermittently applying broad surface area compression and heat sinking; and  
         [0020]    [0020]FIG. 10 is an enlarged fragmentary view of the head of the tool of FIG. 9. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    Referring now to the drawings, particularly to FIG. 1, there is illustrated a combined compression and heat sinking tool generally designated  10  in conjunction with a welding heat source generally indicated  12 . The welding heat source may be any type of fusion welding heat source; for example, electric arc, laser beam or electron beam and with or without added weld material. The heat source  12  is illustrated forming, with or without added weld material, a weld pool  14  eventually forming a weld bead  16 . The weld bead  16  may be applied at the juncture of two adjoining parts to secure the parts one to the other, or may be provided in an overlying manner to a surface to provide a cladding. It will be appreciated from a review of FIG. 1 that the direction of the welding is in the direction of the arrow  18 .  
         [0022]    Tool  10  includes a multiple element compression tool  21  comprising a plurality of pins or needles  20  carried by a housing  22 . By means not shown, the needles or pins  20  are mounted for multiple repetitive reciprocating movement such that the pin or needle heads  24  repeatedly impact the surface of the hot weld nugget while the weld material holds a temperature within a predetermined temperature range in which the weld material is sensitive to stress-crack formation. Thus, as the weld progresses in the direction of arrow  18 , the pins or needle heads  24  repeatedly hammer or peen the surface of the hot weld nugget to plastically deform the surface of the hot weld material as the strain level of the stress/strain curve moves into the plastic region. The near surface material of the weld thus goes into the plastic region of the stress/strain curve and contains the compressive stress supplied by the pins or needles  20 . The needles or pins may be reciprocated by any known means such as electrical, mechanical or fluid, e.g., pneumatic or hydraulic means.  
         [0023]    In order to accommodate the shape of the weld, the housing including the needles or pins  20  may be displaced, for example, oscillated in a direction generally normal to the direction of welding. Thus, the pin or needle heads  24  may be oscillated side-to-side to provide direct perpendicular impacts on the surface of the weld material  16  where the weld pool has an arcuate exposed surface as illustrated. It will be appreciated that purge gas is typically utilized in conjunction with the welding torch or heat source  12 . A curtain  26  is disposed between the heat source  12  and the pins or needles  20  to confine the purge gas in the area of the welding torch.  
         [0024]    In order to provide heat sinking simultaneously with the compressive impact of the pins or needle heads  24  on the surface of the weld material  16 , the needle or pin heads form a conductive heat sink for the weld material. Cooling flows may be provided through passages within the individual pins or needles to cool the heads. Thus, the weld material is simultaneously cooled as the multiple element compression tool  21  repeatedly impacts the surface of the weld material.  
         [0025]    In FIG. 2, a similar arrangement of a combination compression and heat sinking tool with welding heat source is provided and wherein like reference numerals are applied to like parts as in the preceding embodiment preceded by the numeral  1 . In this embodiment, the housing  122  is extended to incorporate the heat source, i.e., the welding torch  112 . Thus an integrated compression cooling and weld assembly is provided. Consequently, the welding torch  112  and the compression and heat sinking tool  110  move jointly in the direction of the weld  118 .  
         [0026]    Referring now to FIG. 3, there is illustrated a mobile apparatus for a combined compression and heat sinking tool for the welding torch for use in an underwater environment. In this embodiment, like reference numerals are applied to like parts as in the embodiments of FIG. 1 preceded by the numeral  2 . In this embodiment, the apparatus includes a housing  222  closed at its top side and ends and having a central opening  202  along its lower face within the confines of the sides and ends of the housing  222 , the applicator head  204  attached to the housing  222  includes a heat source  212  comprised of a welding torch  206  and a coupling  208  for supplying purge gas to the opening  202  in the region of the weld material. The housing  222  also carries a plurality of needles or pins  220  similar to the pins  20  and  120  of the prior embodiments having pin or needle heads  224  which extend through the opening  202 . The pins  220  conduct heat away from the weld head and may have internal passages for flowing a cooling fluid to carry the heat away from the weld head.  
         [0027]    The housing  222  also includes a plurality of individually or independently movable fingers  211  on opposite sides of the housing  222  and which fingers are pivotable about generally parallel axes. Each finger  211  includes a large radius tip  215  and is spring-biased by springs, not shown, into an extended pivoted position to engage a work surface. It will also be appreciated that as the tool  210  is displaced; for example, along a groove and in the direction of the groove, i.e., parallel to the longitudinal extent of the fingers  211 , the fingers  211  follow the contour of the working surface. Also, the side walls and fingers of the housing form an exclusion area inhibiting egress of water into the opening and enabling operation of the torch. The pin or needle heads  224  are mounted for repeated reciprocating movement so that the heads impact the surface of the hot weld material simultaneously while the heads cool the weld material.  
         [0028]    Referring now to the embodiment hereof illustrated in FIG. 4 wherein like reference numerals are applied to like parts as in the embodiment of FIG. 1 preceded by the reference numeral  3 , there is illustrated a housing  310  mounting pins or needles  320  having heads  324  for impacting and cooling the surface of the weld material. The welding heat source  312  is illustrated separate from the housing  310  but may be integral therewith as illustrated in FIG. 2. The needle or pin motion, i.e., transverse oscillatory motion relative to the direction of welding, is similar to that described with respect to FIG. 1.  
         [0029]    In FIG. 4, however, the cooling effect of the heads  324  of the pins  320  is augmented by a following cooling distribution flexible disk  330 . The disk is mounted for rotation in a supplementary housing  332  which may be attached to housing  310 . In this form, a liquid coolant  334  is supplied to the housing  332  and into the wheel or disk  330 . Additionally, a constant pressure force is applied to the housing  332  in the direction of the arrow  336  such that the disk  330  applies a continuous and constant pressure on the surface of the weld material at a location substantially immediately following the location of the impacts of the pin or needle heads  324  on the surface of the weld material. This cooling augmentation is provided while the temperature of the weld material lies within the range of temperature during welding which typically leads to stress-cracking in the resulting weld.  
         [0030]    Referring now to FIG. 5, wherein like reference numerals are applied to like parts as in the embodiment of FIG. 1 preceded by the numeral  4 , the welding torch or heat source  412  forms the molten pool  414  similarly as in FIG. 1. However, instead of compressing the welding material employing multiple impact pins or needles, the weld material is repetitively pounded or hammered by multiple pins  440  mounted on a drum  442 . The pins  442  are mounted for repetitive axial extension and retraction; for example, by internal springs. The movement of the housing  444  carrying the drum  442  is transversely oscillatory in the direction of the arrow  446  as the housing advances in following movement relative to the heat source  412 . The housing  444  may also be mounted for vertical reciprocating movement in a direction normal to the weld surface. Thus, the weld surface is dimpled by the pins  440  and the motion of the wheel  442  provides for an overlapping of the dimpling effect in the surface of the weld material to compress the weld material. The multiple pins  440  carried by the wheel  442  also provide conductive cooling to the weld surface.  
         [0031]    Referring now to FIGS. 7 and 8, there is illustrated a further embodiment of the present invention wherein like reference numerals are applied to like parts as in the embodiment of FIG. 1 preceded by the numeral  5 . In this form, the compression and heat sinking tool  510  follows the weld heat source  512  similarly as in the prior embodiments. The housing  522  mounts a disk  550  for rotational movement about an axis transverse to the direction of movement of the tool  510 . The central portions of the disk  550  include a hard conductive material for planishing the weld material. Straddling the cylindrical surface of the hard planishing material are compliant rims  554 . Rims  554 , as illustrated in FIG. 8, receive a coolant  556  by way of a coolant inlet  558  to housing  522  and wheel  550 . The central portion  552  of the wheel  550  is formed of a hard material which provides a heat sink for the hot weld material. In addition, the annular rims cool the margins of the weld material and distribute a cooling effect to the central portion of the wheel. In this form, the housing has an applied downward force indicated by the arrow  560 . As the housing  522  moves in the direction of the weld, the planishing wheels deform the weld material plastically while simultaneously cooling the weld material in conjunction with the coolant provided by the annular rims. As in the embodiment of FIG. 1, the tool  510  may be oscillated in a transverse direction relative to the direction of travel of the tool along the weld head.  
         [0032]    Referring now to the embodiment illustrated in FIGS. 9 and 10, wherein like reference numerals are applied to like parts as in the embodiment of FIG. 1 preceded by the numeral  6 , there is illustrated a heat source  612  for providing a molten weld pool. The compression and heat sinking tool  610  in this form comprises a shoe  660 . The shoe is in the form of a receptacle containing a coolant supplied to the housing  662  and shoe  660 . The shoe  660  has a elongated lower contact surface  664  which mounts a plurality of pins  670  fixed to the surface. The housing  662  may be reciprocated in a direction normal to the weld bead as indicated by the arrow  668  as the weld progresses in the direction of the weld pass. The housing  662  may also be provided with an oscillatory reciprocating motion in the direction of the weld, i.e., an oscillatory motion about a transverse axis  671  as well as a side-by-side oscillatory motion similarly as in the preceding embodiments. It will be appreciated, therefore, that the pins  670  mounted on the bottom of the contact surface  664  repeatedly impact the surface of the weld material. Additionally, a coolant is supplied through the housing  662  to the shoe  660 . Thus, the heat sinking can be preformed simultaneously with the compressive action of the pins  670  on the weld surface.  
         [0033]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.