The metal properties after welding depend on the alloy AND the quenching process, if any. Remember, the only difference between annealing and quenching is the alloy and the cooling process. It is quite easy to greatly LOWER the strength of the entire structure by employing seat of the pants "normalizing".
Welding and heat treatment are a science, WHEN DONE CORRECTLY. What Bill said about hydrogen is spot on, and I haven't heard anyone mention that in at least three years.
Unless you are willing to invest enough time and effort to get well beyond the "bit of knowledge is a dangerous thing" phase, I would strongly advise utilizing the services of a proven professional for this type of service.
Also, remember that metal alloys have changed over time, not because the specs have changed, but because steelmakers can more closely match specified minimum alloy content(s), and thus reduce their costs, while incurring small risk of producing off-quality steel that is out of spec. They used to have to be much more generous to make sure they ended up well within spec.
