WASHINGTON — The Federal Reserve is keeping a key interest rate unchanged in light of global pressures that risk slowing the U.S. economy. As a result, Fed officials expect to raise rates more gradually this year than they had envisioned in December. The officials now foresee two, rather than four, modest increases in their benchmark short-term rate during 2016. The Fed said Wednesday that the economy has continued to grow moderately but that the global economy and financial markets still pose risks.