There is a difference between a visitor walking through a plant and getting injured because of a trip, choking on a piece or candy, etc. and a person that is asked to perform "work" that is not an employee or a contractor.
A contractor usually has to have insurance as a condition of getting a contract to perform work on-site. That insurance covers the contractor's employees should an accident occur and an injury results.
While the laws vary from state to state, the workers compensation system is nearly the same in every state. Basically the employee gives up the right to file a legal claim against the employer. In return, the employee gets immediate compensation if an injury is sustained on the job. There is no need to hire a lawyer or wait until the claim has been processed by the court system.
If a worker sustains an injury that can be attributed to a third party (someone other than the employer), the injured party can initiate legal action. Once the case is heard by the court and a payment is made, the injured party has to pay back any monies collected from the workers compensation system. Even if the worker sustains an injury that is attributed to the actions or inactions of the employer, the employee can rarely initiate legal action against the employer unless there is some really outlandish actions taken by the employer.
Test the welders as they walk in the door as a condition of employment if you want to, but I'm telling you in clear simple language that they will own a piece of your company if they have half a brain, a good lawyer, and they are injured while taking the welder performance test.
Before discounting what I have to say, call your insurance carrier and check to see if a job applicant is covered for injuries sustained while taking your test before being placed on the payroll. It is better to check and verify you are protected than to find out your insurance doesn't cover you after the accident occurs. More than likely it isn't even a toll call.
Many of my clients will not allow me to touch a piece of equipment, move test samples, etc. while I'm in their facility because of the legal implications should I get hurt as a result. It isn't a union issue, it is an insurance issue.
Fault HR if you want to, but many of them have been schooled on this subject. My daughter's sister-in-law is the Director of HR for a large multinational company and some of the stuff she tells me is an education in what can and cannot be done legally.
Best regards - Al